PARENT SESSION
Poster Session # 17: Aquatic Systems.
Thursday, August 7 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.
Phytoplankton photosnthesis, primary production and nutrient limitation in reservoirs along a gradient of watershed land use.
Dickman, Elizabeth*,1, Vanni, Michael1, Renwick, William1, 1 Miami University, Oxford, Ohio
ABSTRACT- Land use can have many effects on aquatic ecosystems. However, little is known about how lake phytoplankton respond to changes in watershed land use. We investigated how reservoir phytoplankton photosynthetic parameters, primary production, and nutrient limitation status vary along a gradient of watershed land use. Our seven reservoirs reside in watersheds that range from mostly forested (80% forest, 13% agriculture) to highly agricultural (89% agriculture, 9% forest). Urban areas represent only a small fraction of land in all watersheds. We found that late summer primary production (mg C/m2/d) was highly correlated with the percentage of watershed land comprised of agriculture (r2>0.8). Maximal (light-saturated) photosynthetic rate of phytoplankton was also correlated with percent agriculture, but the relationship was much weaker than that between primary production and percent agriculture. Phytoplankton were nutrient limited in all reservoirs. Nitrogen (N) was the limiting nutrient is some lakes, while phosphorus (P) was limiting in others; co-limitation by N and P was common. There was no relationship between land use (percent agriculture in watersheds) and either the identity of the limiting nutrient (N versus P), or the severity of nutrient limitation. Our results suggest that agricultural activities in watersheds increase primary production, most likely by increasing nutrient subsidies. However, across the land use gradient, phytoplankton apparently attain a biomass at which they exhaust resources, and thus become nutrient limited.
Key words: land use, nutrient limitation, primary production, watershed