Carbon Fluxes from River to Sea: Sources and Fate of Carbon in a Shallow, Coastal Lagoon

Lagoons act to transport, retain (via sedimentation), and divert (via outgassing) carbon (C) on its route from land to sea. Their role in transporting vs. attenuating C fluxes is important to understanding global C cycles and sources of organic matter supporting food webs. Here, we present a C budget for a large coastal lagoon in the Baltic region that incorporates measurements of river-estuary, estuary-marine, and sediment-water exchanges, along with internal processes (production and respiration) governing transformations among C fractions. Organic C fluxes were dominated by internal cycling (GPP and R), whereas inorganic C fluxes were largely dependent on hydrological transport. Sediment-water exchange of DIC and DOC was of lesser importance, despite the shallowness of the lagoon. On an annual basis, the lagoon was a net source of organic matter (OM) to the Baltic Sea as export of dissolved and particulate fractions exceeded riverine and marine inputs by 37 ± 4%. Export of OM was due to internal production of POC via phytoplankton photosynthesis. We combined the mass balance and metabolism results with a consumer energetics approach to align C sources with C flows through the lagoon food web. We estimate that the annual harvested fish production accounts for nearly 22% of OM inputs from internal and external sources. A comparison with C flux data from the Chesapeake region allowed us to appreciate how ecosystems at the river-estuarine transition differ in their roles as pipes vs. reactors, depending on the sources and timing of OM inputs, and how these differences constrain food web energetics.