An experiment was conducted with six 13-m3 land-based mesocosms (5 m deep) in December 1996/February 1997 to address the impact of increased temperature on the trophic structure of nutrient-rich coastal systems. All mesocosms were exposed to a high nutrient loading rate (2.31 mmol N m23 d21 : 0.18 mmol P m23 d21 : 0.165 mmol Si m23 d21). Three treatment mesocosms were maintained at a temperature elevated ;18C relative to the long-term (1977–1989) average, ambient temperature in the parent system, Narragansett Bay, Rhode Island, and elevated ;38C from 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 zooplankton and the benthos. Mean phytoplankton biomass and abundance were lower in the mesocosms with warmer temperatures. Well-developed phytoplankton blooms occurred in two of the three cool systems. The presence of high numbers of filter-feeding mussels (Mytilus edulis) prevented a bloom from occurring in the third cool system. Unlike most benthic organisms, mussels continue to filter at high rates even at very low temperatures. Analyses of variance (ANOVAs), after adjusting for mussel biomass, revealed significant (P , 0.05) or near significant (P , 0.10) differences in phytoplankton (abundance and biomass), zooplankton abundance, and sedimentation rates between warm and cool treatments. Experimental and literature data were combined to develop carbon budgets for the six systems. Budgets for the warm systems indicated that carbon produced by phytoplankton was lost primarily by grazing of zooplankton, mussels, or both (29–55%) and to a lesser degree, by sedimentation (29–43%). In the cool systems without mussels, losses via sedimentation (73–82%) predominated, with an average ninefold increase in the amount of material supplied to the benthos relative to warm systems. The seasonal cycle of phytoplankton biomass in temperate regions is typically dominated by a winter-spring phytoplankton bloom and, to a lesser extent, one in the fall (Cushing 1959; Smayda 1973a). CO2 production by our global industrial society has altered the earth’s climate, with increased warming occurring in most regions of the northern hemisphere over the past century (Manabe and Stouffer 1994; Schuurmans 1995). Correlative evidence indicates that increased winter water temperature, resulting from climate warming, may affect the size of the winter-spring phytoplankton bloom in temperate coastal areas (Oviatt 1994). Since the fate of the photosynthetically produced organic matter can significantly influence the trophic structure of marine systems, it is important to understand the relationship 1 Present address: U.S. Environmental Protection Agency, Tarzwell Drive, Narragansett, Rhode Island 02882.