Differences in nutrient limitation and grazer suppression of phytoplanktonin seepage and drainage lakes of the Adirondack region, NY, USA

1. For seepage and drainage lakes of the Adirondack mountain region (NY, U. S.A) hydrologic regime is correlated with physical and chemical differences that can affect phytoplankton and planktonic food webs (e.g. presence and influence of wetlands, dissolved organic carbon concentration, anoxia, nutr ient cycling). We conducted short-term (48 h), in situ enclosure experiment s to evaluate the relative importance of macrozooplankton grazing and nutri ent limitation of phytoplankton biomass in small Adirondack seepage and dra inage lakes (N = 18, 1-137 ha). Epilimnetic dissolved organic carbon (DOC) concentrations and pH values represented the diversity of the region. We me asured chlorophyll a changes in response to grazer removal (> 120 mu m) and nutrient addition (similar to 10x ambient N, P, or N + P), and evaluated c hanges with respect to in situ light, temperature, NO3, NH4, SRP, and crust acean assemblage characters. 2. Nutrient addition stimulated significant increase in chlorophyll a conce ntration at 11 of 18 sites (GLM, Tukey-Kramer). Phytoplankton of clearwater drainage lakes were P-limited, whereas clearwater and brownwater seepage l akes responded to additions of N and/or N + P. Relative light availability explained half the variance in response to nutrient addition in drainage (r (2) = 0.48), but not seepage lake experiments (P > 0.05). 3. We observed responses to grazer removal at eight of 18 sites, usually cl earwater drainage lakes. Crustacean grazing may be as significant as nutrie nt limitation of [chl a ] for many drainage lake phytoplankton assemblages. Responses were related to in situ density of zooplankton only in drainage lakes. Light explained some variability in response to grazer removal for d rainage (r(2) = 0.35) and seepage lake experiments (r(2) = 0.35). 4. These experiments provide evidence that hydrology may ultimately play an important role in determining nutrient and grazer regulation of phytoplank ton. Proximate mechanisms affecting our results may be associated with diff erences in wetland vegetation, [DOC], and nutrient cycling.