Algal biomass and nutrient data were gathered at 322 randomly selected sites on 261 streams in the West Fork White River, Whitewater River, East Fork White River, Upper Wabash River, Kankakee River, Lower Wabash River, Tributaries to the Great Lakes, and Tributaries to the Ohio River Basins in Indiana from May through October for years 2001 through 2005. Basin characteristics (land use and drainage area), substrate, turbidity, and nutrient concentrations were determined for the basin and sampling sites. The relations of the seasonal algal biomass parameters periphyton chlorophyll a (CHLa), ash-free dry mass (AFDM), seston CHLa, and particulate organic carbon (POC) to concentrations of the seasonal nutrients nitrate, total Kjeldahl nitrogen (TKN), total nitrogen (TN), and total phosphorus (TP) were determined using Spearman’s rho. The effects of streamflow were determined using data collected at U.S. Geological Survey (USGS) streamflow-gaging stations spatially located throughout the study basins.

Throughout the 5-year study, the magnitude and frequency of stream discharge varied monthly and annually and greatly influenced algal biomass concentrations through scour and algal drift. Algal biomass median concentrations in Indiana streams consisted of periphyton CHLa, 41.2 milligrams per square meter (mg/m² ); AFDM, 52.1 grams per square meter (g/m²); seston CHLa, 2.44 micrograms per liter (µg/L); and POC, 0.75 milligrams per liter (mg/L). Approximately 32 percent of the periphyton CHLa and 6 percent of the seston CHLa samples would be considered eutrophic (nutrient enriched).

To ascertain seasonal variability, samples were collected in the spring (May), summer (June through August), and fall (September through October). The highest median concentration of periphyton CHLa was in the spring, 63.2 mg/m² , while the highest median concentrations of AFDM, seston CHLa, and POC were in the summer 55.4 g/m² , 2.96 µg/L, and 0.81 mg/L respectively. There were no significant differences among seasons for periphyton CHLa and AFDM; there were significant differences among seasons for seston CHLa and POC.

There were no significant relations with nutrients and periphyton or seston CHLa parameters. The only significant positive relations were observed between summer POC and summer TP as well as summer POC and summer TKN. Positive relations also related spring POC and spring TP. These significant relations with TP are most likely related to phosphorus associated within seston algal cells and attached to sediment.

Drainage area and land use were analyzed to understand the effect of site location on algal growth. Study basins varied in size (headwater streams, 0–51 km² ; wadable streams, 52–2,590 km² ; and boatable streams, 2,591–38,900 km² ) and were dominated by agricultural land use. Basin characteristics (land use, drainage area) as well as substrate type, turbidity, and nutrients, affected the concentration of algal biomass parameters. Of the eight basins in which samples were collected during the 5-year study, the Whitewater River Basin (2002) had the highest median concentration of periphyton CHLa (63.1 mg/m² ), the Tributaries to the Great Lakes (2005) exhibited the highest median concentration for AFDM (160 g/m² ), the East Fork White River Basin (2002) had nearly twice the median concentration (4.01 µg/L) of seston CHLa as the other basins, and the West Fork White River Basin (2001) exhibited the highest median concentration of POC (1.10 mg/L). Of the eight major basins sampled, 15–45 percent of the periphyton CHLa and up to 20 percent of the seston CHLa samples were eutrophic. Samples collected at headwater and wadable streams were the most eutrophic for periphyton CHLa (31–36 percent) and 28 percent of samples collected at boatable streams were eutrophic for seston CHLa.

As basin size increased, seston CHLa and POC concentrations increased while periphyton CHLa and AFDM concentrations decreased. The median turbidity values ranged from 6.95 NTU for headwater streams to 8.27 NTU for wadable streams, and 17.0 NTU for boatable streams. In addition, the types and availability of periphytic substrates (epilithic, epipsammic, or epidendric) were an important factor when comparing periphyton CHLa and AFDM concentrations in the study due to the periphytic substrates individual susceptibility to bed movement and scouring. Periphyton CHLa median concentrations ranged from 53.8 mg/m² for epilithic substrates, to 41.8 mg/m² for epipsammic substrates, and 17.2 mg/m² for epidendric substrates. Higher AFDM concentrations were collected from epipsammic substrates during years of low stream discharge velocity, which enhanced the settling of organic matter on epipsammic substrates. AFDM median concentrations ranged from 141 g/m² for epipsammic to 28.8 g/m² for epilithic and 22.9 g/m² for epidendric substrates. 

The seasonal values for nutrients (nitrate, TKN, TN, and TP) and algal biomass (periphyton CHLa, AFDM, seston CHLa, and POC) were compared to published U. S. Environmental Protection Agency (USEPA) values for their respective ecoregions. Algal biomass values either were greater than the 25th percentile published USEPA values or extended the range of data in Aggregate Nutrient Ecoregions VI, VII, IX and USEPA Level III Ecoregions 54, 55, 56, 71, and 72. If the values for the 25th percentile proposed by the USEPA were adopted as nutrient water-quality criteria, then about 71 percent of the nutrient samples and 57 percent of the CHLa samples within the eight study basins would be considered nutrient enriched.