Bivalve carrying capacity in coastal ecosystems

carrying capacity of suspension feeding bivalves in 11 coastal and estuarine ecosystems is examined. Bivalve carrying capacity is defined in terms of water mass residence time, primary production time and bivalve clearance time. Turnover times for the 11 ecosystems are compared both two and three dimensionally. Fast systems, e.g., Sylt and North Inlet, have turnover times of days or less, while, slow systems, e.g., Delaware Bay, have turnover times of months and years. Some systems, Marennes-Oléron, South San Francisco Bay and North Inlet, require a net influx of phytoplankton in order to support their bivalve populations. Three systems, Carlingford Lough, Chesapeake Bay and Delaware Bay, have very long bivalve clearance times due to small or reduced bivalve filter feeder populations. Carlingford Lough stands out because it is a naturally planktonic system now being converted to bivalve culture with an adherently stronger benthic-pelagic coupling. Existing models of bivalve carrying capacity are reviewed. The Herman model is utilized as an appropriate ecosystem level model to examine carrying capacity because it includes the three major turnover time elements of water mass residence time, primary production time and bivalve filter feeder clearance time. The graphical analysis suggests that massive and successful bivalve filter feeder populations are found in systems with relatively short residence times (<40 days) and short primary production times (<4 days) in order to sustain a high bivalve biomass with its associated rapid clearance times. Outlier systems are constrained by long water mass residence times, extended primary production times, and long clearance times. [KEYWORDS: bivalves, ecosystems, carrying capacity, turnover time, mussels, oysters]

A review of the feedbacks between bivalve grazing and ecosystem processes

This paper gives an overview of interactions between bivalve grazing and ecosystem processes, that may affect the carrying capacity of ecosystems for bivalve suspension feeders. These interactions consist of a number of positive and negative feedbacks. Bivalve grazing can result in local food depletion, which may negatively influence bivalve growth. On a larger scale, it may induce a top-down control of phytoplankton biomasss, and structural shifts in phytoplankton composition. In the case of harmful algal blooms, phytoplankton may negatively affect bivalve grazing rates. The processing of large amounts of particulate matter may change nutrient cycling on the scale of estuaries, and can result in changes in the inorganic nutrient pool available for phytoplankton, through regeneration and reduced storage of nutrients in algal biomass. This can reduce nutrient limitation of the phytoplankton and stimulate algal growth rates. Observations from mesocosm studies suggest that a positive feedback from bivalve grazing on phytoplankton growth may also change the physiological state of the algae and improve food quality. [KEYWORDS: suspension-feeding bivalves, phytoplankton, nutrient cycling, primary production, carrying capacity]