No abstract

High resolution profiles of methane concentration and potential methane-oxidation rates were used to test the importance of in situ production and oxidation in controlling methane structure and dynamics in the southern anoxic basin of the Pettaquamscutt Estuary, Rhode Island, USA. Profiles conducted in both April and August showed low but increasing methane concentrations in the surface oxic layer, a minor maximum about 0.75 m above the Eh = 0 boundary, a methane minimum just above Eh = 0, and a rapid increase to a major maximum about 0.75 m below Eh = 0. Methane concentrations in both maxima increased between April and August while concentrations in the methane minimum decreased. This methane minimum in the suboxic zone was strongly associated with a minor oxygen maximum and a major peak in chlorophyll due to the oxygenic alga Euglena proxima. April incubations in the presence of added oxygen at 2°C supported oxidation rates as high as 9% d-' in and just below the methane minimum at the Eh = 0 boundary, due to psychrophilic methane-oxidation. Similar experiments conducted in August at 22°C showed a similar narrow peak in potential methaneoxidation rates in and just below the methane minimum at the Eh = 0 boundary, suggesting that the boundary was now dynamically controlled by rapid mesophilic methane-oxidation. Analysis of the August incubations at 2°C showed a decline in methane at the depths of the major methane maximum for 4 d, followed by a 4 d increase in methane concentration at the same depths, due presumably to the exhaustion of dissolved oxygen and the cessation of the masking oxidation. These results indicate that the seasonal increase in the major methane maximum is generated Ln situ by psychrophilic methaneproduction, a production that IS largely masked by a concomitant psychrophilic methane-oxidation. The shift from an initial dominance of methane-oxidation to a final dominance of methane-production in the same water sample bottles indicates just how sensitive the structure of the major methane maximum is to the bacterial processes of methanogenesis and methanotrophy, changes in water temperature, and oxygen inputs by in situ primary production and physical intrusions. Such processes have made it bfficult to document a major production of methane in the pycnocllne.

ABSTRACT:Vertical mixing processes and resulting physical structures may influence dynamics of marine ecosystems. Knowledge of the ecological significance of mixing processes has been restricted by our lack of ability to control and manipulate these factors experimentally. A technique is described that permits development and maintenance of 2-layered stratification in land based mesocosms. A variety of vertical physical structures and mixing regimes within and between layers can b e simulated which are typical of various parts of the marine environment. In an initial experiment, replicate, stable multi-trophic level model ecosystems developed and maintained themselves for 2 mo. The biological replicability and ability of the system to support multiple trophic levels including zooplankton and ctenophore predators over long periods of time suggests that these systems have onsiderable promise for analysis of effects of mixing and spatial heterogenity on the dynamics of coastal marine ecosystems.