Limnetica 32

Ecosystem metabolism is an integrated descriptor of lake functioning. In systems dominated by submerged vegetation, as in many coastal environments, estimates of whole-system metabolism that are calculated through free-water diel techniques can be compromised by the high spatial heterogeneity of the metabolic signal. We investigated the spatial variability in the dissolved inorganic carbon (DIC) and dissolved oxygen (DO) concentrations and in the derived ecosystem metabolism in a coastal lagoon dominated by dense meadows of canopy-forming macrophytes. We analysed the gross primary production (GPP), respiration (R) and net ecosystem production (NEP) from diel variations of both DIC and DO at different sites during the period of maximum activity of the meadows.

Our results showed high spatial variability in the DIC and DO concentrations in the vertical and horizontal dimensions as a result of the intense metabolic activity of macrophytes in the littoral surface waters. High heterogeneity in metabolism was also observed in the lagoon, with a mean coefficient of variation of up to 48 % for GPP and R rates. Most of this variability occurred within the littoral areas with macrophytes. The DIC-derived metabolic rates were systematically higher than the DO-derived rates (slope 2.074), indicating the existence of strong inorganic carbon fluxes. Our results stress the need for high sampling efforts based on multiple sampling sites and coupling of DIC and DO estimates to allow accurate quantification of ecosystem metabolism in shallow lakes and lagoons that are dominated by submerged vegetation.