My research background is in zooplankton ecology, and copepods (Class Crustacea) in particular. These small organisms are a critical component of aquatic food webs, and are among the most important metazoans by biomass on the planet. While my research has ranged from the level of individual gut cells (Hassett and Blades-Eckelbarger 1995) to that of ecosystems (Hassett et al. 1997), the ‘red thread,’ as a former professor would say, that runs through this research is that of food supply. How do zooplankton respond to variations in the quantity and quality of their food supply?
The Role of Feeding History in Feeding Behavior
The focus of my thesis research was the role that feeding history plays in subsequent feeding behavior, looking at time–scales ranging from hours to weeks (Landry and Hassett 1985). A copepod’s food supply varies substantially as a function of daily vertical migration, intermittent storms, and seasonal weather patterns. By measuring the response of the planktonic marine copepod Calanus pacificus’ feeding rate, digestive enzyme activities, and assimilation efficiency to changes in food supply over these time scales, I gained an insight into the importance of different scales of food supply variability in the life history of the copepod (Hassett and Landry 1982, 1983, 1988, 1990a, 1990b; Landry et al. 1984).
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Current research at the Mount Desert Island Biological Laboratory
My post-graduate research was directed towards island zooplankton ecology, with research on copepod egg production in the Aleutian Archipelago (Hassett et al. 1993) and temporal and spatial patterns of copepod distribution around the island of Oahu in the Hawaiian Archipelago (Hassett and Boehlert 1995, 1999).
Gut-Cell Morphology in Relation to Feeding
Additional post-doctoral work in the lab of Darcy Lonsdale at SUNY-Stony Brook, done in collaboration with Pamela Blades-Eckelbarger of the Darling Marine Center of the University of Maine, investigated diel patterns in physiology and gut cell morphology in the copepod Acartia tonsa, utilizing feeding rate measurements, digestive enzyme assays, and image analysis of copepod sections under light microscopy and TEM. Acartia is a coastal copepod that displays a diel cycle in feeding despite the continuous presence of food. The aim of this study was to understand the role that changes in gut cells play in this cycle, and to determine if the cycling of these cells is an integral part of the digestive process in copepods. In contrast to Calanus, a genus that experiences diel changes in food supply due to vertical migration and which shows diel changes in gut cell morphology, Acartia demonstrated little change in gut cell morpholgy over a diel cycle (Hassett and Blades-Eckelbarger 1995). I have done additional research in this area with Darcy Lonsdale and Fred Dobbs that relates changes in physiology and gut cell morphology in the harpacticoid copepod Coullana canadensis to an apparent reproductive-resting stage in the copepod (Lonsdale et al. 1998).
Ecological Stoichiometry in Marine and Pelagic Ecosystems
Beginning in 1992 I expanded my research into the limnological world in a collaborative study with James Elser of Arizona State University involving the effect of zooplankton chemical composition on nutrient cycling (an area called 'ecological stoichiometry'). Differences in the proportions of nitrogen to phosphorus between zooplankton and their food will result in different proportions of these elements being retained and recycled by the zooplankton. This project was a comparative study of the characteristics of a variety of aquatic and marine ecosystems, with data obtained on 30 lakes and 21 marine stations. We found striking differences between marine and freshwater systems in how N and P are apportioned, with marine systems being in closer stoichiometric balance than lakes, indicating that the potential for zooplankton to affect the N:P balance through nutrient recycling is far greater in freshwater systems (Elser and Hassett 1994; Elser et al. 1995; Hassett et al. 1997). Another significant result was our finding that Daphnia, the common freshwater cladoceran, was most abundant in lakes with a low N:P ratio (i.e., relatively phosphorus rich), which is likely due to the high nutritional demand of Daphnia for elemental P.
Other Recent research
More recently funded research projects involved 1. effects of toxic algae (the 'red-tide' dinoflagellate Alexandrium) on copepod physiology (Hassett 1997; Hassett 2003) and 2. development of a protocol using sensitive fluorometric assays to determine biochemical compostion and activities of several digestive and metabolic enzymes (including an assay for citrate synthase and carnitine palmitoyltransferase, Hassett and Crockett 2000). This method is being applied to a study of the physiological condition of Calanus finmarchicus CV copepodites in the Gulf of Maine. Calanus finmarchicus CV's are often observed in two different states, a torpid, lipid-rich state and an active, lipid-poor state. Analysis of metabolic and digestive enzymes of individual CV copepodites from the Gulf of Maine demonstrates distinct differences in metabolic potential between the two groups (manuscript submitted). However, within each group there is also considerable variation, indicating that individuals are in varying stages of dormancy.