Technical Memo

Wenck Associates, Inc. | 7500 Olson Memorial Highway | Golden Valley, MN 55427

Toll Free 800-472-2232 Main 763-252-6800 Email wenckmp@wenck.com Web wenck.com

To: North Fork Crow River Watershed District (NFCRWD)

From: Tom Langer, Biologist, Wenck Associates, Inc.

Jeff Strom, Associate, Wenck Associates, Inc.

Date: 16 August 2017

Subject: Grove Lake Early and Late Season SAV Surveys

Introduction

Grove Lake is located near the city of Grove Lake on the edge of Pope County, MN. Grove

Lake has an approximate surface area of 344 acres, with 230 littoral acres (areas less than

15 feet deep), 5.13 miles of shoreline, and a maximum depth of 31 feet. Wenck Associates,

Inc. was contracted by the NFCRWD and the Grove Lake Association to conduct aquatic

vegetation point intercept surveys to document the extent of early spring curlyleaf

pondweed and the late summer vegetation community (native and AIS).

The 2017 survey efforts focused on the eastern basin of the lake. The eastern basin is

deeper than the western basin and has significantly more lakeshore properties. The western

basin is shallow with few residential properties. Efforts were made to document the species

present in the western basin, however, late season navigation was difficult and not fully

assessed. This technical memorandum summarizes the survey methodology and results of

the recent assessments.

Survey Methodology

A well vegetated littoral area promotes and facilitates the health of a lake’s ecosystem by

providing critical spawning, foraging, and nursery habitat for aquatic biota. The littoral area

is also an important area for recreational activities and aesthetics to lakeshore property

owners. This assessment specifically focused on littoral areas 15 feet deep or less

throughout the lake. Wenck staff (Tom Langer and Jeff Strom/Sarah Nalven) conducted

modified point intercept field surveys on May 25th, 2017 and July 31st, 2017. Wenck did not

sample the shallow western basin during the July survey due to thick vegetation coverage,

however, part of the basin was navigated and species and relative abundances were noted.

No AIS were observed and similar species to the eastern basin dominated the western

basin.

A double sided weighted 14 tine rake was thrown from the boat and retrieved across the

lake bottom to represent approximately 0.5 square meters of vegetation sampling. We refer

to this as a rake toss. For each rake toss, vegetation was identified to the species level,

placed in a perforated bucket, weighed, and assigned a proportion of the total biomass

based on visual approximation (i.e. 80% of total weight was curly-leaf pondweed). All

biomass values are reported in wet weights (kg). At each rake toss locations, a corrected

sonar depth reading was also recorded.

Continuous sonar readings were also collected during the survey trip using a Lowrance HDS

Sonar/ GPS unit. This data was processed using ciBioBase software

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(https://www.cibiobase.com/) to map vegetation biovolume. Biovolume differs from

biomass in that it provides context to vegetation water column density. The higher the

biovolume the more saturated the water column is with vegetation. Since the sonar

readings are collected continuously, ciBioBase processing also helps delineate and

interpolate the presence/absence of vegetation and vegetation biovolume between rake toss

locations.

Point Intercept Results

May 25th, 2017

A total of 195 survey points (184 littoral points) were assessed within the main basin with

14 species observed (Table 1). Muskgrass, coontail, and Canadian waterweed were the

most frequently observed species within the lake. Curlyleaf pondweed was the only AIS

observed within the lake and was observed across the eastern basin with no occurrence in

the western basin. The densest areas of curlyleaf pondweed was observed in the north and

eastern part of the eastern basin (Figures 1 and 2).

Table 1: Species and occurrence during May 25th 2017 sampling event.

Common Name Scientific Name Total

Observations Percent Littoral

Occurrence

muskgrass Chara sp. 97 53%

coontail Ceratophyllum

demersum 81 44%

waterweed (Canadian) Elodea canadensis 61 33%

duckweed (star) Lemna trisulca 42 23%

water milfoil (northern) Myriophyllum sibiricum 41 22%

yellow waterlily (small) Nuphar microphyllum 41 22%

curly-leaf pondweed Potamogeton crispus 30 16%

floating pondweed Potamogeton natans 26 14%

white-stemmed pondweed Potamogeton praelongus 24 13%

clasping leaf Pondweed Potamogeton richardsonii 22 12%

flat-stemmed pondweed Potamogeton zosteriformis 21 11%

white water buttercup Ranunculus aquatilis 9 5%

Bladderwort greater Utricularia vulgaris 7 4%

water celery Vallisneria americana 1 1%

July 31st, 2017

A total of 123 survey points (120 littoral points) were assessed within the main basin with

14 species observed (Table 2). Northern watermilfoil, muskgrass, and coontail, were the

most frequently observed species within the lake. No AIS were observed during this survey.

Curlyleaf pondweed senesces typically in June and therefore is rarely observed during late

season surveys. An abundance of native species were observed within the basin and with

relatively high biomass (Figure 3). Many of the survey points in relatively shallow water

depths (<3-4ft) were not accessible by boat due to the density of the vegetation. Though

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these areas were dense, the species that appeared to persist in these areas were either 1) a

mixture of native species, or 2) relatively monodominant muskgrass stands, both of which

are represent healthy community stands within the basin. Due to navigational difficulties in

the highly vegetation areas, the ciBioBase sonar processing poorly modeled biovolume and

therefore was not include in our final analyses (Figure 3).

The MnDNR has developed a Floristic Quality Index (FQI) to rate the health of the

vegetation community within a lake. Grove Lake’s FQI score during the July survey was

29.4 which is well above the state impairment threshold (score = 18.6) for lakes in this

ecoregion. These results suggest that, despite the presence of curlyleaf pondweed and high

vegetation density in areas, the plant community in Grove Lake is relatively healthy.

Table 2: Species and occurrence during July 31st 2017 sampling event.

Common Name Scientific Name Total

Observations Percent Littoral

Occurrence

water milfoil (northern) Myriophyllum sibiricum 68 57%

muskgrass Chara sp. 52 43%

coontail Ceratophyllum demersum 52 43%

Bladderwort greater Utricularia vulgaris 38 32%

Illinois pondweed Potamogeton illinoensis 32 27%

yellow waterlily (common) Nuphar variegata 22 18%

waterweed (Canadian) Elodea canadensis 21 18%

water celery Vallisneria americana 19 16%

duckweed (star) Lemna trisulca 18 15%

flat-stemmed pondweed Potamogeton zosteriformis 18 15%

white-stemmed pondweed Potamogeton praelongus 13 11%

clasping leaf Pondweed Potamogeton richardsonii 8 10%

sago pondweed Stuckenia pectinata 6 7%

large-leaved pondweed Potamogeton amplifolius 1 5%

GROVE LAKEMay 25th 2017 - Curlyleaf Pondweed Biomass Figure 1

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Date: 8/16/2017 Time: 3:38:31 PM User: lanta0700

AUG 2017

Curlyleaf PondweedBiomass (kg/m2)D 0.00

0.01 - 0.250.26 - 0.500.51 - 0.750.76 - 1.00

GROVE LAKEMay 25th 2017 - Curlyleaf Pondweed Biomass (Eastern Basin) Figure 2

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AUG 2017

Curlyleaf PondweedBiomass (kg/m2)D 0.00

0.01 - 0.250.26 - 0.500.51 - 0.750.76 - 1.00

Biovolume100%

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GROVE LAKEJuly 31st 2017 - All Species Biomass Figure 3

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AUG 2017

All SpeciesBiomass (kg/m2)D 0.00") 0.01 - 0.50") 0.51 - 1.00") 1.01 - 2.50") >2.5

Survey Area