Allometric scaling of estuarine ecosystem metabolism

Abstract

<p id="d4321955e133">Coastal margins host some of the most productive habitats on the planet, yet there is still significant uncertainty about the magnitude of carbon fluxes across the land/ocean interface. Scaling up in situ measurements into whole-ecosystem estimates of how these habitats contribute to global biogeochemical cycling has been hindered by their apparent heterogeneity, complex circulation, and sizes that range from tens of meters to hundreds of kilometers. Here, I show that estuarine ecosystem metabolism varies predictably with ecosystem size. The significance of this study is not simply that larger estuaries have lower specific ecosystem metabolism than smaller estuaries, but that this size scaling arises because the residence time of limiting nutrients does not scale isometrically with estuary size. </p><p class="first" id="d4321955e136">There are still significant uncertainties in the magnitude and direction of carbon fluxes through coastal ecosystems. An important component of these biogeochemical budgets is ecosystem metabolism, the net result of organismal metabolic processes within an ecosystem. In this paper, I present a synthesis of published ecosystem metabolism studies from coastal ecosystems and describe an empirical observation that size-dependent patterns in aquatic gross primary production and community respiration exist across a wide range of coastal geomorphologies. Ecosystem metabolism scales to the 3/4 power with volume in deeper estuaries dominated by pelagic primary production and nearly linearly with area in shallow estuaries dominated by benthic primary production. These results can be explained by applying scaling arguments for efficient, directed transport networks developed to explain similar size-dependent patterns in organismal metabolism. The main conclusion from this synthesis is that the residence time of new, nutrient-rich water is a fundamental organizing principle for the observed patterns. Residence time changes allometrically with size in pelagic ecosystems because velocities change by only an order of magnitude across systems that span more than ten orders of magnitude in size. This nonisometric change in velocity with size requires lower specific metabolic rates at larger ecosystem sizes. This change in transport may also explain a shift from predominantly net heterotrophy to net autotrophy with increasing size. The scaling results are applied to the total estuarine area in the continental United States to estimate the contribution of estuarine systems to the overall coastal budget of organic carbon. </p>