Factors limiting benthic algal abundance in Virginia streams of the

Coastal Plain

Michael Brandt Paul A. Bukaveckas

Virginia Commonwealth University

Center for Environmental Studies

Why study benthic algae in streams?

• Widely used as bioindicator of nutrient impairment – assumes that benthic algal abundance is determined by nutrient availability.

• However, other factors influence abundance.

Factors influencing benthic algal abundance:

Incident solar radiation (canopy effects) nutrient availability grazing substrate stability

Substrate effects may be particularly important in

Coastal Plain streams which are characterized by

fine, unstable substrates (sand, clay, silt).

Substrates and benthic algae:

Fine substrates may limit the abundance of benthic algae even in nutrient-rich streams due to mortality effects associated with washout and burial.

Question:

How important is substrate composition in determining inter-stream variation in benthic algal abundance in streams of the Coastal Plain?

Methods Survey: benthic algal abundance measured

monthly for 1 year among five streams differing in substrate composition, riparian cover and nutrient availability.

Experimental: algal biomass accumulation measured on hard substrates (ceramic tiles) vs. naturally occurring substrates (fine benthic matter) in two streams differing in substrate composition (high vs. low proportion of hard substrates).

ReachesStream Order

Substrate Composition

CanopyTributary

River

Crump 2nd97% Sand 3% Hard

Moderate York

Courthouse 2nd64% Sand36% Hard

Dense James

Powell 3rd78% Sand22% Hard

Moderate James

Herring 3rd10% Sand90% Hard

Moderate James

Kimages Reference

2nd Sand, Silt, Clay* Moderate James

Kimages Restored

2nd Clay, Sand, Silt* Open James

Study Sites

“Hard” = Pebble, gravel or rock; * qualitative substrate survey only

Measured Variables:

Algal biomass (as CHLa) – measured at ten locations along a 200 m study reach at each of five sites.

Incident Solar Radiation (PAR) - measured at same locations as for CHLa.

Nutrients (DIN, SRP) – measured at top/bottom of each study reach; determined colorimetrically using a Skalar segmented flow analyzer (APHA et al 1992).

Substrate Composition - % cover by substrate types (gravel, sand, etc.) estimated within 0.5 m² frame placed at five random spots at each sampling location (as for CHLa and PAR).

Results

Pooled data (all sites) show positive relationship between benthic algal abundance and substrate composition.

Results

Light and nutrient availability accounted for only a small proportion of variation in benthic algal abundance in Coastal Plain streams.

Site Y XModel

r²p-value

Pooled Data CHLaSubstrate, SRP, Discharge,

PAR0.80 <.0001

Regression Analysis

A multivariate model that included substrate type, nutrient concentrations, discharge and light availability as explanatory variables accounted for 80% of the variation in benthic algal abundance.

Experimental Component

CHLa on Tile (Artificial Substrate, mg/m²) CHLa on Natural Substrate (mg/m²)Powell Creek 80.6±23.9 56.6±13.4Herring Creek 29.9±5.2 61.1±14.3

At the site dominated by hard substrates (Herring) algae were more abundant on natural substrates than artificial substrates. At site dominated by fine substrates (Powell) algae were more abundant on artificial substrates.

Conclusions Stable substrates which included hard materials (gravel,

etc.) and aggregated clay were associated with higher levels of benthic algal biomass. These sites likely provided greater stability for algal colonization relative to sand and silt, resulting in lower mortality from scouring and sedimentation.

Incident solar radiation and nutrient concentrations were secondary factors affecting algal abundance in Coastal Plain streams.

With constraints imposed by the lack of hard substrates, how does benthic algal abundance in Coastal Plain streams compare with other VA regions ?

Comparison of benthic CHLa of streams in the Piedmont and mountain regions of Virginia (data from VADEQ) with data from this study (Coastal Plain).

Implications for biomonitoring:

Streams with larger proportion of unstable substrates (sand) are likely to exhibit lower levels of algal biomass at comparable light and nutrient levels relative to streams where hard substrates are more prevalent.

Low algal biomass in sandy streams is not necessarily indicative of low nutrient levels.