Biotic Turnover Research Articles

A simulation of saltmarsh water column dynamics

This deterministic model of water column dynamics in saltmarsh tidal creeks focuses mainly on mechanisms of carbon and nitrogen cycling within the water. The model includes biotic interactions among phytoplankton, zooplankton, dissolved inorganic nitrogen, detritus and dissolved organics. In addition, the model examines the effects of tidal exchange with the nearshore coastal ocean. In simulation experiments, the model isolates the water column submodel from simulated vegetated marsh surface, oyster reefs, and subtidal benthos. By comparing simulation results with field data from the North Inlet saltmarsh (Georgetown County, SC) we examined the relative importance of water column processes in controlling seasonal changes observed in the tidal creeks. Results suggest that for the components with rapid biotic turnover rates (phytoplankton and dissolved inorganic nitrogen) much of the observed seasonal pattern is controlled by internal cycling within the water column (planktonic productivity, nutrient uptake and remineralization). For components with slower turnover rates (zooplankton, detritus, and dissolved organic matter) tidal exchange with the coastal ocean plays a larger role in controlling seasonal variability. The model accounts for less of the observed variability in detritus and dissolved organic matter, suggesting the seasonal importance of upland runoff and exchanges with other marsh subsystems in controlling concentrations in tidal creek.

Primary production, isotopes, extinctions and the atmosphere

Phytoplankton photosynthesis has controlled the atmospheric carbon dioxide-oxygen balance since the early Precambrian when algal abundance became sufficient to convert the reducing atmosphere to an oxygenic one. Later phytoplankton maxima indicated in the geologic record coincide with and are here suggested to have caused extensive biogenic calcareous deposition, and evolutionary diversification of the contemporaneous biota. In contrast, the onset of decreased productivity in the geologic past triggered a general biotic turnover on both land and sea. Times of phytoplankton extinctions coincide with similar periods affecting animal taxa rather than terrestrial plants. These periods of severely reduced microfloras and low productivity resulted in atmospheric oxygen depletion and increased pCO 2, 12C enriched limestones, formation of 32S enriched sulfates, and submarine dissolution of carbonates. Selective extinctions of animal taxa were due to quantitative variations in the interrelated processes of carbon fixation (the food source for the marine environment), and the photosynthetic consumption of carbon dioxide and production of oxygen. The phytoplankton abundance may have been controlled by contemporaneous continental physiography, through its effect on climate, atmospheric circulation and oceanic upwelling. Low continents, equable climates, little influx of land-derived materials and lessened effectiveness of upwelling allowed the sinking of nutrients to exceed their renewal, resulting in marked reduction of phytoplankton, with the attendant effects of the decreased productivity and oxygen depletion crossing ecologic and systematic boundaries. Continental rejuvenation with resultant climatic changes and increased oceanic circulation stimulated renewed phytoplankton diversification and growth, and allowed a renewed expansion of dependent animal taxa.