Predicted impacts of elevated temperature on the magnitude of the winter-spring phytoplankton bloom in temperate coastal waters: A mesocosm study

An experiment was conducted with six 13-m(3) land-based mesocosms (5 m deep ) in December 1996/February 1997 to address the impact of increased tempera ture on the trophic structure of nutrient-rich coastal systems. All mesocos ms were exposed to a high nutrient loading rate (2.31 mmol N m(-3) d(-1):0. 18 mmol P m(-1) d(-1):0.165 mmol Si m(-3) d(-1)). Three treatment mesocosms were maintained at a temperature elevated similar to 1 degrees C relative to the long-term (1977-1989) average, ambient temperature in the parent sys tem, Narragansett Bay, Rhode Island, and elevated similar to 3 degrees C fr om three control mesocosms. Warmer temperatures were hypothesized to result in lower phytoplankton biomass during the winter-spring bloom period as a result of increased grazing related to greater metabolic activity of both z ooplankton and the benthos. Mean phytoplankton biomass and abundance were l ower in the mesocosms with warmer temperatures. Well-developed phytoplankto n blooms occurred in two of the: three cool systems. The presence of high n umbers of filter-feeding mussels (Mytilus edulis) prevented a bloom from oc curring in the third cool system. Unlike most benthic organisms, mussels co ntinue to filter at high rates even at very low temperatures. Analyses of v ariance (ANOVAs), after adjusting for mussel biomass, revealed significant (P < 0.05) or near significant (P < 0.10) differences in phytoplankton (abu ndance and biomass), zooplankton abundance, and sedimentation rates between warm and cool treatments. Experimental and literature data were combined t o develop carbon budgets for the six systems. Budgets for the warm systems indicated that carbon produced by phytoplankton was lost primarily by grazi ng of zooplankton, mussels, or both (29-55%) and to a lesser degree, by sed imentation (29-43%). In the cool systems without mussels, losses via sedime ntation (73-82%) predominated. with an average ninefold increase in the amo unt of material supplied to the benthos relative to warm systems.