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).

Reaches Stream Order Substrate Composition Canopy Tributary River Crump2 nd 97% Sand 3% Hard ModerateYork Courthouse2 nd 64% Sand 36% Hard DenseJames Powell3 rd 78% Sand 22% Hard ModerateJames Herring3 rd 10% Sand 90% Hard ModerateJames Kimages Reference 2 nd Sand, Silt, Clay*ModerateJames Kimages Restored 2 nd Clay, Sand, Silt*OpenJames 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.

SiteYX Model r² p-value Pooled DataCHLa Substrate, SRP, Discharge, PAR 0.80<.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 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.