Organic carbon patterns and budgets in the Long Island Sound estuary

Abstract

AbstractA multi‐year observational time series was evaluated across the 150 km central axis of the U.S. east coast's Long Island Sound (LIS) estuary, in three distinct regions. Fluxes were calculated at the boundaries of the regions using observations coupled to a hydrodynamic model and applied to a mass balance to assess organic carbon (OC) export from LIS. For all years, during stratified summer periods, LIS was a net exporter of OC to the continental shelf. LIS annual net carbon export however, varied with river flow. The heterotrophic or autotrophic nature of LIS also shifted inter‐annually. During the mass balance analysis period (2009–2012), LIS ranged between net OC import from the continental shelf and heterotrophy in the lowest river flow year (2012) and net export of OC and autotrophy in the highest flow year (2011). Analysis suggests that LIS switches from net OC import to export when the annual river inputs exceed 19 km3 yr−1. Applying these thresholds to the annual river flow record suggests that net import occurred in 15% of the last 20 years and that LIS usually is a net exporter of OC (85%). Annually averaged LIS carbon export values based on river flow conditions over the last 20 yr are estimated at 56 ± 64 × 106 kg yr−1. Analysis also suggests that LIS shifts from net heterotrophic to net autotrophic when annual river flow exceeds 26 km3 yr−1 (35% of the last 20 yr). Net heterotrophic conditions are most common, representing 65% of the last 20 yr.