Subtidal Ecology of Frenchman Bay

Establishing long-term monitoring sites J. Alex Brett

College of the Atlantic

Spring, 2011

[2]

Abstract Long-term monitoring programs are an essential component of understanding

population dynamics, particularly in response to anthropogenic activity. The marine

environment of Downeast Maine is subject to a wide range of factors resulting from human

actions, including commercial fishing, climate change, and species introductions. In order to

provide baseline data to understand the impacts of these actions I used photoquadrats to

survey the rocky subtidal zone at two sites in Frenchman Bay, Maine. The data demonstrated

interesting trends in population distribution patterns in the Bay; I found high levels of

variation both between sites and between individual quadrats within sites. The site at Bar

Island had a significantly higher density of filamentous red algae than Long Porcupine. As

predicted by past studies, percent cover estimates of filter feeders were found to vary

significantly between horizontal and vertical surfaces. The data from this study support

earlier conclusions about marine invertebrate population patterns, and indicate the

importance of fixed plots for long-term surveys to reduce the effect of environmental

variation on survey results.

Introduction

At present there appears to be little current understanding of the population dynamics

of the benthic marine life of Downeast Maine, which is a knowledge gap that needs to be

addressed. The bulk of population studies on marine life have focused on commercially

important species, such as scallops (Hart 2006) and bony fishes (Brodziak and Traver 2006,

Mayo and O’Brien 2006), or ecologically sensitive species such as eelgrass (Disney and

Kidder 2010). From a management standpoint this is inadequate, because NOAA has been

advocating the implementation of ecosystem based fisheries management for more than ten

years (NMFS 1999), which requires a near-complete understanding of the ecosystem (Link

2002). On an even more fundamental level, long-term ecological data is an essential tool for

understanding changes in the natural world (Strayer et al. 1986, Mikkelsen and Cracraft

[3]

2001, Magurran et al. 2010), and it is a tool that is all too often ignored in favor of studies

providing immediate results.

Due to the difficulty of observing organisms underwater, estimates of marine

populations have primarily relied on indirect methods, such as trawls or landing data from

commercial fisheries (Pearson and Barnett 1987, Chen et al. 2005, Brodziak and Traver

2006). When combined with models, these can give a ballpark figure for populations, but

since there are never direct observations of the organisms in question, the models are not

completely reliable (NRC 2000, Rose and Cowan 2003). The indirect survey methods used

by fisheries managers are generally conducted with the same gear used in the fisheries

(Kilduff et al. 2009), which means that species of no commercial importance will generally

not be accounted for in the surveys. These indirect methods are generally destructive to the

organisms being studied, and given the current overstressed state of the ocean (Bax et al.

2003, Myers and Worm 2003, Feely et al. 2004), nondestructive survey methods are

preferable.

When designing a long-term monitoring program, it is important to make sure that the

area being studied is being accurately represented. If a particular portion of a habitat is not

included, that may have dramatic impacts on the results of the study. In the rocky subtidal,

not all areas of rock are created equal; horizontal and vertical surfaces can have a very

different population of species (Sebens et al. 1999, Miller 2005). The reasons for this are

varied, but two major factors are sedimentation and quantity of light that reaches the ocean

bottom (Miller and Etter 2008). In order to properly understand a rocky subtidal site through

a long-term monitoring program, the differences between horizontal and vertical rock faces

should be studied and accounted for by a diversity of microhabitats.

[4]

The data that has been gathered in Frenchman Bay to date has given little focus to the

rocky subtidal zone. There has been some work on historical fisheries (Alexander et al. 2009)

and dredgeable surfaces in the subtidal (Procter 1933), but data around the rocky subtidal is

scarce. A major factor in this may be that remote monitoring of such a habitat can be

difficult, because of the expense of using a remote operated vehicle (ROV) and because of

the inefficiency of more conventional sampling gear on rocky surfaces. To this end, in the

following study I have attempted to create a baseline dataset on the invertebrate community

of Frenchman Bay using direct observation by SCUBA divers; these data can be used to

examine small-scale distribution patterns in algae and invertebrates. In addition the data will

allow future researchers to study long-term changes in population.

Methods of Censusing Marine Populations

Early methods of sampling and quantifying marine organisms relied on a range of

indirect methods, such as dredges and grab samples (Robson 1923, Procter 1933). These

methods were generally effective, and are still in common usage (Pearson and Barnett 1987).

However, there are two major issues with indirect sampling that makes more direct methods

preferable.

First, indirect sampling is destructive to the organisms being studied and to the

environment they inhabit. There have been concerns about damage caused by trawls almost

as early as they were first used (Jones 1992). In a number of areas, chronic trawling has led

to large shifts in the structure of benthic communities (Watling and Norse 1998, Jennings et

al. 2001, Thrush and Dayton 2002). It should be noted that these are the results of heavy

[5]

commercial fishing, and research trawls do not approach that level of frequency, though they

may still cause damage at low intensity.

Additionally, indirect methods can be unreliable in a number of ways when compared

to direct methods of observation (Lessios 1996). Catch rates for some methods, such as

dredges, can be quite low; on the order of 11% for scallops (Mcloughin et al. 2003), and 2-

32% for oysters (Chai et al 1992). This level of variation can make it difficult to compensate

for the low catch rates and would likely lead to inaccurate population estimates. Another

factor to be considered is that some habitats are more favorable to trawling, and when species

are non-randomly distributed between habitats there can be a bias in the population estimates

(Jagielo et al. 2003). Trawls and other indirect methods are useful tools, but when possible

direct, nondestructive observation should take preference.

Remote operated vehicles (ROVs) provide in situ observations of marine life down to

the deepest regions of the world’s ocean (Bowen et al. 2009). Unlike trawls and other

indirect survey methods they can provide direct evidence on the abundance of organisms as

well as behavioral information. There is of course the potential for an observer effect on the

organisms being studied (Spanier et al. 1994, Stoner e al. 2008), which could impact both the

behavior and the numbers being reported. To date, the majority of ROV work has been

conducted at depths beyond the range of compressed air SCUBA diving (eg. Krieger 1993,

Newman and Stakes 1994, Reed et al. 2005, Kahng and Kelley 2007). However, recently

they have been used more frequently at depths safely reachable by SCUBA divers (Lirman et

al. 2006, Lame et al. 2007). ROVs do not run the risk of decompression sickness or other

pressure related injuries that divers must be wary of, which makes them an excellent choice

[6]

for surveys from a safety perspective. However, there are several factors that currently make

ROVs unsuited for most shallow water operations.

One of the primary disadvantages of using ROV for surveys of marine life is their

high cost. A simple model rated to 150 m from Seabotix Inc. starts at $29,995, and when

extra features are added that price could easily rise by $50,000 or more (Stephanie Arbridge,

Advex Marine, 2011, pers. comm.). A similar ROV system from Videoray, another large

manufacturer of ROVs for research and commercial use, starts at $43,495 (Paul Igo,

Oceanographic and Geophysical Instruments, 2011, Pers. Comm.). Equipment can be leased

to avoid the high costs, but costs can still be in the neighborhood of $1,000 a day for a

relatively basic model (Pacunski et al. 2008). An additional complication with ROVs is the

high level of logistical support needed during operation (Pacunski et al. 2008). Some recent

work has shown ways for small ROVs to be operated with a minimum of surface support

(Csepp 2005), but they still generally require more logistical support than operations using

SCUBA. As technologies develop and advance, the price of ROVs may decrease and

handling operations may become easier, but at present those difficulties generally restrict

ROVs to surveys beyond the depth of air-based SCUBA divers.

Like ROVs, SCUBA diving allows scientists to directly observe target organisms

without needing to kill or damage them first. Additionally, diving-based operations are

relatively cheap compared with the expense of renting or purchasing an ROV. However,

SCUBA diving has its own set of restrictions that limits its usefulness for research. Dive-

based surveys are incredibly time consuming in comparison with trawl or dredge-based

studies. Diving also involves an increased danger to the participants, which means that safety

parameters must be built in to prevent harm and that further reduces the amount of time that

[7]

can be spent conducting a survey (AAUS 2006). Like many other methods, the presence of

the diver can induce an observer effect in the organisms being studied, though that is mostly

of concern for highly mobile organisms such as fishes (Dickens et al. 2011).

There is no one best technique for censusing marine populations using SCUBA. Each

environment and study organism has its own set of quirks that require fitting research

methods to individual project locations. Thus, any attempt to summarize all methods would

either fail or become an unreadable tome, so I will settle for a brief comparison of some of

the more common techniques. The three primary categories that most surveys fall into are

video, photographic, and visual. Visual surveys generally require the least equipment beyond

basic SCUBA gear, which makes them attractive for low-budget operations or situations

where large numbers of volunteers are engaged in data collection (Koss et al. 2005). They

also are generally more accurate than videos or photographs (Murray 2001, Tremblay et al.

2005). However, they can be time consuming and require all researchers to be highly familiar

with the marine flora and fauna of the region (Murray 2001).

In comparison with visual surveys, photographic and video-based censuses tend to be

more costly, because of the additional electronics required. However, they have advantages

that can make the added expense worthwhile. Photographs provide a permanent record of the

area being studied (Sebens et al. 1997), which allows additional analysis to be performed at a

later date. Digital images also allow the area covered by encrusting organisms to be

calculated from digital photographs with analysis software, such as Image J (Lirman et al.

2006), which is generally the most accurate method for determining percent cover (Meese

and Tomich 1992). Other methods, such as stereophotography, have been successfully used

for obtaining measurements from photography (Christie 1980, Lundalv and Christie 1986),

[8]

but unless three-dimensional measurements are required, analysis from a single digital

photograph is a simpler technique. Aside from the high initial investment, photography also

has the disadvantage that it can be difficult to impossible in areas of high algal overgrowth

(Bullimore and Hiscock 2001). Surveys based around photography offer a good compromise

between time spent in the water and accuracy of data.

With its increased affordability, researchers are turning to underwater video more

frequently for subtidal censuses (Carleton and Done 1995, Munro 2001, Lirman et al 2006).

Like photography, it permits large areas to be surveyed while expending relatively little

bottom time, but it also requires large amounts of time post-dive for analysis (Leonard and

Clark 1993). Another potential disadvantage to video is that it can be difficult to correctly

locate and identify small cryptic organisms (Leonard and Clark 1993) or organisms in

complex habitats (Tremblay et al. 2005). Identification down to the species level can also be

difficult for morphologically similar animals, such as corals (Ninio et al. 2003). One

advantage that video shares with photography is that divers do not need to be familiar with

the organisms being censused (Munro 2001), which allows for a broader pool of assistants to

be employed in data collection. Based on the potential difficulty of identifying small

organisms, video surveys seem to be best suited for studies on larger mobile invertebrates or

to quantify general characteristics of a habitat.

Survey Site and Potential Threats

Frenchman Bay is located in Downeast Maine just East of Mount Desert Island and

the bulk of Acadia National Park. There was a strong fishery for Atlantic cod (Gadus

morhua) and other gadids in the bay well into the 19th

century (Alexander et al. 2009). More

[9]

recently, the bay has been used extensively for American lobster (Homarus americanus)

fishing along with a number of other fisheries. In the 1990’s the green sea urchin

(Strongylocentrotus droebachiensis) fishery began to grow throughout Downeast Maine,

including Frenchman Bay, but by 2001 the fishery had crashed and landings statewide were

less than a quarter of what they had been (Hunter et al. 2005, Edward Monat, 2011, Pers.

Comm.). During the same period, draggers in the Bay began harvesting orange-footed sea

cucumbers (Cucumaria frondosa) and blue mussels (Mytulis edulis) (Edward Monat, 2011,

Pers. Comm.). Currently, Frenchman Bay is the site of the bulk of the fishing effort for

orange-footed sea cucumber for the whole Gulf of Maine (Chen et al. 2005). Additionally,

clam and mussel harvesters from around the state have converged on upper Frenchman Bay,

because it is one of the few locations in the state that is not closed to harvesting because of

red tide events during midsummer (Trotter, 2009). All of these activities have presumably

had a range of effects on the ecology of the bay, whether through the direct removal of

organisms from the water or the modification of habitat with fishing gear. Though there is

some evidence to indicate a shift in ecology of the bay as a result of fisheries and other

human activies (Edward Monat, 2011, Pers. Comm.), there is no solid data to describe what

exactly has changed. While it would be most helpful to have data from before these intensive

fisheries, it is still important to gather data now to understand any future changes that may

arise from the fishing effort in the bay.

Aside from commercial fishing there are global and regional scale issues that could

have powerful impacts on the ecology of Frenchman Bay. Current predictions estimate a

global temperature rise of 0.3° C per decade over the next hundred years, which has the

potential to cause significant shifts in marine ecosystems (IPCC 2007). Another component

[10]

of climate change that is often overlooked is ocean acidification; over the next hundred years

ocean pH is predicted to drop by 0.3 to 0.4 units, an increase of 100 to 150% in H+

concentration (Orr et al. 2005). A decrease in pH could have profound effects on the

phytoplankton and zooplankton that form the base of the marine food chain, as well as larger

organisms such as shellfish and snails (Orr et al. 2005). Although it would have been ideal to

have data starting before the industrial revolution, it is essential that we gather long-term

population data to understand the impacts that anthropogenic climate change is having on the

ocean.

In addition to climate change, the benthic ecology of Frenchman Bay also faces the

potential arrival of several marine invasive species. The Asian shore crab, Hemigrapsus

sanguineus, is spreading throughout Southern Maine, but as of several years ago only a

single specimen has been observed as far East as Frenchman Bay (Delaney et al. 2007). They

are of particular ecological concern because of their potential to outcompete other crabs (Ahl

and Moss 1999, Jensen et al. 2002). Asian shore crabs are found mainly in the intertidal, but

Codium fragile, an invasive green algae, is quite common subtidally from the New

Hampshire border to Blue Hill Bay, just to the west of Frenchman Bay, though no specimens

have been found in Frenchman Bay (Chris Petersen pers. comm.). In the regions farther south

where it has been introduced, Codium fragile has disrupted shellfish communities and

outcompeted native kelp species (Hart 2005). Given the apparently imminent arrival of these

species it is important to establish baseline data on the community as it exists currently, so

that we can understand the changes that they may bring.

[11]

Figure 1: Map of MDI and Frenchman Bay. Black crosses represent the survey sites. Bi=Bar Island. LP=Long

Porcupine. Source: USGS.

Methods

Rocky subtidal communities were surveyed at two survey sites located along the

Porcupine Islands, a chain that runs East-West across Frenchman Bay, Maine, USA (Figure

1). The sites off of Bar Island and Long Porcupine Island are at a depth of 12-15 meters and

the bottom consists of a series of rocky ledges rising out of the mud and gravel. These

locations were chosen based on ease of access with a small rigid inflatable combined with

recommendations from a local commercial diver.

Bi

LP

[12]

Initial random surveys were carried out at two sites, and a third site is scheduled for

inclusion in the survey when the permanent plots are added. The random survey consisted of

photographing 20-30 randomly placed 0.1 m2 quadrats per site and later counting all

invertebrates and algae in each image. Half the quadrats were placed on horizontal substrates

and the other half were on vertical surfaces to better understand the differences between the

two habitat types. Horizontal surfaces were anything with a slope of less than 30 degrees and

vertical surfaces had a slope of at least 60 degrees. A quadrapod built from aluminum angle

stock along with a Sealife DC800 camera with a single strobe was used for the photography.

Examples of the photographs are given in figures 2 and 3.

For the permanent survey, six to eight 0.1 m2 quadrats will be marked and

photographed using the same photographic techniques. The quadrats will be randomly placed

on rock ledges, with half placed on vertical surfaces and half on horizontal. Originally, the

quadrats were planned to be 0.25 m2

based on the work of Witman (1985) and Lundalv and

Christie (1986), but after testing with several different quadrat sizes it was decided that 0.105

m2 (30cm x 35cm) would allow for better identification. The quadrats will be marked with

two brightly painted 316-grade stainless steel rods at the diagonally opposite corners of the

quadrat. The marker rods will be epoxied into holes made in the rock with a pneumatic drill.

The aluminum quadropod built for the random survey will align with the site markers to

ensure exact positioning each time the survey is conducted.

Four 10 m x 1 m transects will be swum for each site to quantify abundance of larger

mobile invertebrates, such as lobsters (Homarus americanus), rock crabs (Cancer irroratus),

and jonah crabs (Cancer borealis). In order to properly represent the lobster population, the

transects will be swum in midsummer, when the

[13]

lobsters are inshore for spawning. Each end of the

transect lines will be marked with a stainless steel

rod fastened by epoxy, and a line will be strung

between them each time the survey is conducted.

The transects will be placed randomly within

“edge” areas that provide good habitat for lobsters

and crabs. The combination of the transects and

photoquadrats is based on a long-term monitoring

program in Massachusetts (Sebens et al. 1997), but

with a larger quadrat size to better represent larger

invertebrates.

Once the permanent plots are established,

the following protocol will be used. The divers will

begin the dive by locating the transect endpoints

and stringing the transect line. In order to prevent

bias in the transect counts from the presence of the

divers, they will then locate and photograph the

marked quadrats. After all of the quadrats have

been photographed, the divers will swim the transects

with a 1 meter length of PVC to delineate the boundaries of the transect. Any large mobile

invertebrates with more than 50% of their body inside the transect will be counted.

The photographs were uploaded to a computer and analyzed in two ways. All

individual invertebrates in the photographs large enough to be identified were counted and

POLY Polysiphonia sp.

Filamentous red algae

DIDA Didemnum sp.

White encrusting tunichate

HALP Halichondria panicea

Crumb of bread sponge

COR Coralline crust

(Lithothamnion

glaciale/Phymatolithon

rugolosum)

MUD Mud

DEB Debris/dead algae etc.

ROCK Bare rock

HYDR Hydroids in family

Campanulariidae

MICROP Microciona prolifera

Red beard sponge

METR Metridium senile

Frilled anemone

HYDRA Hydractinia echinata

Snail fur

CHOND Chondrus crispus

Irish sea moss

FICUS Suberites ficus

Fig sponge

ERBRYZ Erect bryozoans.

MYSTBRYZ Unidentified bryozoan.

Orange/red, Erect and

encrusting forms.

Table 1: Categories used for the

estimation of percent cover.

[14]

Measured. Invertebrates are considered solitary if they grow independently of others of their

own species. Percent cover of algae and sessile invertebrates was then estimated by

projecting a network of random points across every photograph using the software Photogrid.

Due to difficulties with precise identification from the pictures, some organisms were lumped

into functional groups (Table 1), similar to Sebens (1986). Invertebrates were identified using

Pollock (1998) and algae was keyed out with Villalard-Bohnsack (2003). These data will

allow for comparisons not only between sites and between habitat aspects, but also to analyze

the variability on a microscale between similar habitats within a site.

[15]

Figure 2: Photograph of a rock wall from Bar Island. The plot is primarily dominated by Polysiphonia sp.and

there are two scarlet psoluses in the bottom center.

Figure 3: Photograph of horizontal substrate at Long Porcupine. A clump of campanularid hydroids fills the

top left, and northern sea star (Asterias rubens) and frilled anemone (Metridium senile) occupy the lower right.

[16]

Results

Overview

The two sites were quite similar in

terms of overall bottom composition; both

locations consist of a series of ledges rising

out of a flat muddy bottom, and both are

protected from the open ocean by the

Porcupine islands. At both locations bare

rock was a rarity; almost all surfaces were

either covered in life or in thin layer of

sediment or debris. Larger mobile organisms

such as lobsters and crabs were occasionally

observed in the mud surrounding the ledges,

and no fishes were seen during the course of

the study. Toad crabs (Hyas coarctatus),

which are generally found in rocky subtidal

areas, were too quick to be captured by the

photoquadrats, but were observed around the

study area.

Throughout all the quadrats at both sites, a total of 21 species/groups were counted.

Of the species encountered, eight were mobile invertebrates, three were solitary sessile

invertebrates, eight were colonial invertebrates, and three were algae. The average .105m2

Scientific Name Common Name

Myxicola infundibulum Slime worm

Psolus fabricii Scarlet psolus

Henricia sanguinolenta Blood star

Asterias rubens Northern sea star

Cucumaria frondosa Orange-footed sea cucumber

Metridium senile Frilled anemone

Pagarus acadianus Acadian hermit crab

Flabellina verrucosa Red-gilled nudibranch

Infraorder Doridacea Dorid nudibranch

Order Stauromedusae Stalked jelly

Aeolidia papillosa Maned nudibranch

Table 2: List of scientific and common names of

all solitary invertebrates counted in the

photographs.

[17]

quadrat was 30% covered by living organisms and had a total of 4.15 species. There was a

large range around those averages; the diversity of plots ranged from one species to nine

species and the total coverage by life ranged from 3% to 91%.

Due to the resolution of the images, some species were lumped into general groups or

higher taxonomic categories. The dorid nudibranchs observed were likely Onchidoris

muricata, but there was no way to rule out the possibility that it was another common dorid

such as Adalaria proxima or Acanthodoris pilosa. The situation was the same with the

stalked jelly; it appeared to be Lucernaria quadricornis, but further examination was needed

to differentiate it from the other Stauromedusae found in this region. Erect bryozoans and

campanularid hydroids were lumped together because of the necessity of microscopy for

positive species identification. The hydroids most likely consisted primarily of the genus

Obelia, but there may have been other genera present. Erect bryozoans included Caberea

ellisi and Tricellaria ternate, but it is likely that other species were present.

Variation between sites

The two survey sites were very different in terms of the general community

composition and dominant species. The horizontal substrates at Bar Island were dominated

by the filamentous red algae Polysiphonia sp. with an average percent cover of 48.5%. This

same species was also the most common encrusting organism at Long Porcupine, but at

12.5% or about a quarter of the area covered at Bar Island. On vertical quadrats, the

difference was even more pronounced, with Polysiphonia sp. covering 49.4% at Bar Island

and 6.2% at Long Porcupine. The algae cover between the two sites was significantly

different (p<.001, Mann-Whitney U test statistic=419.5).

[18]

Aside from Polysiphonia sp., there was also a large difference in the population of

filter feeders present at both sites (Figure 4). At Long Porcupine, hydroids were the second

most common group in horizontal quadrats, after Polysiphonia sp., and they were found at

seven out of the 14 surveyed sites. On the vertical plots at Long Porcupine, erect bryozoans

were the most common encrusting organism (Figure 4), yet none were counted at Bar Island.

The percent cover of all filter feeders together was significantly higher at Long Porcupine

(Table 3) (p=.004, Mann-Whitney U test statistic=106.5). Several bryozoans were seen at

Bar Island during dives, but they either did not happen to fall inside a quadrat or they were

not represented by the estimation of percent cover.

Long Porcupine had more diversity of solitary invertebrate life than Bar Island; a total

of ten species compared to seven species. A major part of that difference was the lack of

nudibranchs at the Bar Island site. At Long Porcupine, three species of nudibranch were

observed: Flabellina verrucosa, Aeolidia papillosa, and an unidentified dorid. The only

species found solely at Bar Island was the Acadian hermit crab (Pagarus acadianus). Despite

the overall higher diversity at Long Porcupine, the average density of solitary invertebrates at

Bar Island was higher, at 2.7 m-2

compared with 1.9 m-2

at Long Porcupine. Overall, the two

sites were quite different, particularly in terms of the amount of filter feeders and red algae.

[19]

Figure 4: Percent coverage of encrusting invertebrates and algae on horizontal and

vertical substrates at the two survey sites. The groups are arranged from most

common to least common on vertical substrates at Bar Island to highlight differences

in species composition. A more detailed description of groups/species is given in table

1, but the abbreviations are as follows. POLY=Polysiphonia sp., COR=Encrusting

coralline algae, CHOND=Chondrus crispus, METR=Metridium senile, FICUS=Suberites

ficus, HYDRA=Hydractinia sp., HALP=Halichondria panicea, HYDR=Campanularid

Hydroids, DIDA=Didemnum sp., MYX=Myxicola infundibulum, ERBRYZ=Erect Bryozoan,

MYSTBRYZ=Unidentified bryozoans. Error bars=1 SE.

Variation within sites

There was a large amount of variability among the quadrats within a given survey

site. Histograms of the most densely occurring species, Polysiphonia sp. and campanularid

hydroids are shown in figures five and six. One species, Myxicola infundibulum, was found

25 times in one photograph, four times in another, and then never in any of the other pictures

from Bar Island. Polysiphonia sp. ranged from a coverage of 25 - 82% in the horizontal

0

10

20

30

40

50

60

Pe

rce

nt

Co

ver

Bar Island Horizontal

Bar Island Vertical

Long Porcupine Horizontal

Long Porcupine Vertical

[20]

0

2

4

6

8

10

12

14

0-10 11-20 21-30 31-40 41-50 51-60 61-70 71-80 81-90 91-100

# o

f O

ccu

rne

ces

Percent Coverage

quadrats at Bar Island and from 0 - 42% in the horizontal quadrats at Long Porcupine. Out of

the 30 coefficients of variation calculated for encrusting organisms and algae, only five were

less than 1.0, and nine were greater than 3.0.

Figure 5: Histogram of Polysiphonia sp. coverage.

Figure 6: Histogram of coverage of campanularid hydroids.

0

5

10

15

20

25

30

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

# o

f O

ccu

ren

ces

% Coverage

[21]

Figure 7: Counts of solitary invertebrates from horizontal and vertical substrates at

both survey sites. Error bars equal 1 SE.

Horizontal vs. Vertical

Vertical surfaces generally had a higher density of living organisms than horizontal

surfaces. The coverage of rock by sessile filter feeders was significantly greater on vertical

surfaces at both sites (Table 3) (p=0.011, Mann-Whitney U test statistic=120.5). However,

there was no significant difference between the coverage of algae between horizontal and

vertical surfaces (p=0.762, Mann-Whitney U test statistic=232.5). The overall percentage of

area covered by living organisms increased from 50.4% to 60.1% at Bar Island and from

21.45 to 36.1% at Long Porcupine. In general, solitary invertebrates were less common on

vertical surfaces than horizontal with the exception of the blood star (Henricia

sanguinolenta) at both sites and the slime worm (Myxicola infundibulum) at Bar Island

(Figure 7). Overall, the strongest difference between horizontal and vertical surfaces was the

density of filter feeding organisms.

0.0

5.0

10.0

15.0

20.0

25.0

30.0

De

nsi

ty (

/m2)

Bar Island Horizontal

Bar Island Vertical

Long Porcupine Horizontal

Long Porcupine Vertical

[22]

Table 3: Percent coverage of filter feeders and algae at both sites in comparison with areas of rock,

sediment and debris.

Species Interactions

In order to determine if there

were any significant correlations

between species observed in this

study, Spearman rank correlation

coefficients were calculated for all

combinations of species occurring in

the same plot. There were a number

of significant correlations,

Figure 8: Correlation comparison between percent cover of

Polysiphonia sp. and campanularid hydroids.

but the strongest was the positive relationship with coralline algae and bare rock (p < .01,

Spearman coefficient = .70, n = 42) and the negative relationship between Polysiphonia sp.

and campanularid hydroids (p < .01, Spearman coefficient = -.65, n = 42) (Figure 8). Also

% Algae % Filter Feeders % Bare rock % Mud/debris

Bar Island

Horizontal

49.8 0.6 8.9 37.0

Bar Island Vertical 49.5 11.1 5.9 24.4

Long Porcupine

Horizontal

12.9 9.5 5.3 69.0

Long Porcupine

Vertical

6.2 30.1 3.0 49.6

0

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40

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0 10 20 30 40

% P

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Hydroids

[23]

interesting was the lack of a relationship between campanularid hydroids and either of the

common nudibranchs, Flabellina verrucosa (p > .05, Spearman coefficient = .14, n = 42) and

Aelodia papillosa (p > .05, Spearman coefficient, n = 42). To see if the nudibranchs were

responding to the presence of hydroids rather than the density, I ran a contingency table on

the two variables from the Long Porcupine data set, but the relationship was not significant

(p=0.848. Pearson chi-square statistic=0.037, df=1). So, although nudibranchs were

associated with their food at the level of site, there was not a smaller grained pattern of

association between hydroids and their predators.

Patchy Distribution

Several species showed a tendency to appear in clumps, both inside the quadrats and

elsewhere around the study site, so I tested for non-random distributions in all species.

However, of the species that had occurred frequently enough to be tested, only Asterias

rubens (p = 0.988, KS statistic = 0.069) and Psolus fabricii (p = 1.0, KS statistic = .055) fit

the Poisson distribution, so almost every species was non-randomly distributed. This

indicates that practically all species observed appear in clumps to some degree.

Discussion

The data gathered in this study have provided insight into some population

distribution patterns of benthic invertebrates in the Gulf Maine. Of particular note is the high

level of variability both between survey sites and between photoquadrats within sites. The

high variability highlights the importance of permanent monitoring sites for studying long-

term trends in rocky subtidal environments in this region. The data also supports some

[24]

findings of earlier studies on the different groups that inhabit vertical and horizontal rock

surfaces.

Habitat Variation

The level of variation between the two survey sites was higher than expected given

how apparently similar they are in overall habitat composition and exposure. Both sites were

located on the protected North side of a chain of islands and consist of a series of ledges

rising out of the flat expanses of mud. The only obvious difference between the structure of

the ledges at the two sites is the aspect of the walls. At Bar Island, the ledges run North-

South, so the vertical rock faces are oriented towards the East, and at Long Porcupine, the

ledges run East-West, so they are facing the North. This could affect the amount of light each

wall receives, and since shading is one potential factor structuring rock wall communities

(Miller and Etter 2008), this could be a partial reason for the differences between the two

sites. Another factor that could cause the different community composition between the two

sites is the different flow regime. The Long Porcupine site likely has a much higher average

flow from tidal currents because of the deeper water surrounding the island. This fits with the

pattern of hydroids and bryozoans displaying a preference for increased flow rates (Hughes

and Hugher 1986, Faucci and Boero 2000), since Long Porcupine was where the higher

concentration of filter feeders was found. The differences between the two sites highlight the

importance of incorporating a range of sites into a population study.

Horizontal vs. Vertical Surfaces

[25]

The results of this study support the findings of earlier studies describing the

differences between the communities on horizontal and vertical surfaces (Sebens et al. 1997,

Miller and Etter 2008). Based their findings, the horizontal surfaces were expected to be

dominated by algae, such as Polysiphonia sp., and vertical sites to be dominated by sessile

filter feeders. In this study, sessile filter feeders, such as bryozoans and hydroids, were found

in significantly greater densities on vertical surfaces than on horizontal surfaces, and at Long

Porcupine Polysiphonia sp. was found in higher abundance on horizontal surfaces (Figure 4).

This difference in community composition between microhabitats is potentially the result of

several factors, with sedimentation and light intensity as the two most important (Irving and

Connell 2002, Miller and Etter 2008). Sedimentation has the capacity to smother low lying

filter feeders and low levels of light can prevent algae from growing. Unfortunately data

were not taken on relative light intensity, so it is impossible to say for certain to what degree

the results of this study are a product differential light. However, at Bar Island, Polysiphonia

sp. was about equally abundant on horizontal and vertical surfaces. The most pronounced

difference between the horizontal and vertical surfaces was the increased density of filter

feeders on wall surfaces.

Species Interactions

The strongest correlation between species within plots was the negative relationship

between Polysiphonia sp. and the campanularid hydroids. Without additional habitat

preference data on the two groups it is difficult to guess what is behind that interaction. The

most likely explanation is that some environmental factor, such as sedimentation or flow rate,

varies enough between the two sites and making one site more suitable for the algae and the

[26]

other better for the hydroids. If one looks at the data (Figure 8) it is apparent that although

the correlation is statistically significant, it is not adequately explained by a linear

relationship.

A surprising result of the correlation tests was the lack of a link between the red-

gilled nudibranchs (Flabellina verrucosa) and the campanularid hydroids, because F.

verrucosa feeds on hydroids and was only found in plots where they were present. There

were no nudibranchs present at Bar Island and no hydroids, but at Long Porcupine there were

both nudibranchs and hydroids. Thus, it is possible that the nudibranchs may be responding

to the hydroids on a larger scale than the photoquadrats can measure. If the nudibranchs are

so widely dispersed that they only rarely appear in the plots, the correlation analysis may be

unable to see the relationship between the species. Also, it is possible that the nudibranchs

are merely responding to the presence of hydroids rather than the density of hydroids, in

which case the contingency table test would have demonstrated a relationship, but it did not.

Dominant species

A similar study in the Southern Gulf of Maine found that rock walls with sea urchins

tend to be dominated by them and cleared of any other major species (Sebens et al. 1997).

However, areas with low densities of urchins are dominated by Alcyonium sidereum,

Metridium senile, and Aplidium glabrum (Sebens et al. 1997). Both survey sites in this study

were practically urchin free; no urchins were counted in the photoquadrats, and only a

handful were observed during dives. Based on that, one might expect a similar species

composition here if other factors are the same. However, I found only one of those species,

Metridium senile, and it was generally present in low numbers. Vertical substrates at Bar

[27]

Island and Long Porcupine were either dominated by the filamentous red algae, Polysiphonia

sp., or to a lesser degree by erect bryozoans.

Alcyonium sidereum has shown a preference for high flow, exposed environments,

both between survey sites and within individual rock walls (Sebens 1984). Metridium senile

has also shown a preference for high flow environments through increased rates of pedal

laceration (Anthony and Svane 1994). No similar habitat preference data are available for A.

glabrum or Polysiphonia sp. It seems likely that the different species composition is at least

in part a result of differing exposure and flow regime between the sites, because of the

protected nature of the two survey sites in Frenchman Bay compared with the sites used by

Sebens et al. (1997). There are of course a number of other factors that could be in play,

including the latitudinal difference between the studies.

The lack of green sea urchins (Strongylocentrotus droebachiensis) at the sites in

Frenchman Bay represents another key difference between my sites and the studies done

farther South (Sebens et al. 1997, Miller and Etter 2008). Urchins have the potential to cause

large changes in benthic communities through intensive (Siddon and Witman 2003), so this is

an important factor when considering the long-term population trends of a site. The lack of

urchins at Bar Island and Long Porcupine is likely a result of two major factors. First, both

sites were located a few meters below the lower range of kelp (Laminaria longicruris,

Laminaria saccharina, Agrarum cribrosum), which is a major food source for urchins.

Second is the fact that ten years ago, the urchin fishery in Frenchman Bay collapsed, and

urchin densities have been low since then (Edward Monat, 2011, Pers. Comm.)

Future directions

[28]

This study has begun the work of creating baseline population data for the rocky

subtidal community in Frenchman Bay, and if the survey is replicated it may provide

interesting insights into the population dynamics of marine invertebrates in this region, which

appear to be at least somewhat different than at survey sites in the Southern Gulf of Maine

(Sebens et al. 1997), making it important to have long-term population data from this region

as well. However, to be of most use, the study must be replicated and expanded. Due to the

high variability between and within sites it is important to represent a number of different

habitats in the survey. The addition of several more sites, particularly ones in more exposed

areas, would help the survey provide a better overall picture of population dynamics in

Frenchman Bay. If random surveys are continued, the high variance within sites will make a

high sample size necessary to detect the effects of environmental change. The establishment

of permanent plots would reduce that need, because any change observed would definitely be

the result of changes in the site rather than having sampled a slightly different spot. Thus, I

have two suggestions for the continuation of this research project. First, establish two new

sites in exposed areas. Potential locations are the South side of Burnt Porcupine Island and

South of the Bar Harbor Breakwater. Second, create permanent quadrats using the procedures

outlined in the methods section of this paper. Replicating this survey in the future will help

understand the effects that the large range of anthropogenic factors is having on the

population dynamics of Frenchman Bay.

Acknowledgements: I’m grateful to Steve Ressel, my academic advisor, for all his advice

and support over the past four years. My official project advisor, Chris Petersen, helped

immensely with development of the project and with the revision of the final writeup. My

unofficial project advisor, Eddie Monat, provided invaluable assistance with his local

knowledge and long experience as a commercial diver. The data gathered during this study

will be held by Chris Petersen and Eddie Monat. Without the help of many volunteers, this

[29]

study would never have been completed. Thanks to Zach Whalen, Luka Negoita and

Solomon Spiegel for assistance with diving and to Rowanna Herndon, Ryan Woofenden,

Marina Garland, and Lindsey Erickson for acting as surface support. Boat usage was

generously made possible by Sean Todd and Dan Dendanto of Allied Whale. Toby

Stephenson provided safety oversight for vessel operation. Funding was provided by a Maine

Space Grant Consortium Fellowship.

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