Allometric scaling of estuarine ecosystem metabolism

Abstract

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.

Keywords: allometric scaling; ecosystem metabolism; estuaries; metabolic theory; primary production.

Fig. 1.

(A and B) Gross primary production as a function of (A) surface area and (B) volume. (C) Depth as a function of area. Regression lines in A–C are for the full dataset (orange), systems shallower than 2 m (yellow), and systems deeper than 2 m (blue). Shading indicates 95% confidence interval. Systems listed in C, Right show relative fraction of pelagic gross primary production as a function of depth. Statistics are shown in Table 1. Plots and regressions for community respiration and specific metabolism are in SI Appendix, Figs. S1 and S2 and Table S2.

Fig. 2.

Consider an ecosystem (A) of size $L=\ell$, where $\ell =u\tau$, $\tau$ is an ecosystem-independent growth time, and $u$ is a transport velocity carrying nutrients to and through the ecosystem. The graph depicts the accumulation of organic carbon along the flow path for an ecosystem metabolism of 1 g C per unit time or, equivalently, 1 g C per $\ell$. This corresponds to the service volume, shown in dashed lines. Ecosystems B and C are both twice as long as A, but the velocity in C is 1.25 times larger than the velocity in either A or B. The increased velocity in C results in a lower specific metabolism than in B because the residence time is shorter.

Fig. 3.

NEP as a function of surface area. Regression lines in A and B are for the full dataset (orange), systems shallower than 2 m (yellow), and those deeper than 2 m (blue). Shading indicates 95% confidence interval. Statistics are shown in Table 1.