Understanding variation in zooplankton communities among alpine kettle ponds within the Niwot Ridge LTER

Variation in zooplankton assemblages may reflect large-scale shifts in aquatic communities following changes to the biotic and abiotic environment. Alpine ponds are subject to a wide-range of stressors, including high UV light penetration, nitrogen deposition, and desiccation. Previous studies of alpine kettle ponds have examined zooplankton community response to factors such as predation and UV light, and some studies have specifically investigated pigmentation responses by zooplankton in the subclass copepoda. To understand factors that determine zooplankton community structure and copepod pigmentation, we measured environmental characteristics (i.e., pH, salinity, and temperature), predation, concentrations of total nitrogen and total phosphorus, and dissolved organic carbon (a proxy for UV light penetration) three times for 12 alpine ponds (elevation: 3,100m) near the University of Colorado’s Mountain Research Station. Predator inventories were taken using a dip net followed by enumeration and identification of organisms. Zooplankton collected during horizontal net hauls taken for enumeration, measurement, and identification (to species, when possible). Prior to preservation, copepods from living samples were enumerated and classified according to color into one of three categories: red, blue, or no color. We recorded a total of 26 zooplankton taxa, including 10 cladocera, 4 copepoda, 3 rotifera, 1 ostracoda, 2 oligochaetes, 3 larval insects, and 1 amphibian. Zooplankton body size differed by site (p < 0.001). Environmental characteristics and nutrient concentrations did not affect copepod pigmentation. However, the presence of some predators significantly correlated with the proportion of pigmented copepods. For example, the presence of leeches correlated negatively with the proportion of red copepods (p = 0.04), while the presence of predacious diving beetles correlated negatively with the proportion of blue copepods (p = 0.04). Our results suggest that biotic components of pond communities may be important drivers of site-specific variation in community composition and pigmentation.