Estuarine ecosystem function response to flood and drought in a shallow, semiarid estuary: Nitrogen cycling and ecosystem metabolism

We quantified sediment‐water interface and water column nitrogen (N) transformation rates seasonally over the course of a year that incorporated a major storm event and a period of prolonged riverine base flow in Copano Bay, Texas, a shallow, productive estuary in the western Gulf of Mexico. We calculated daily rates of gross primary production (GPP), community respiration (CR), and net ecosystem production (NEP) in Copano Bay to examine carbon (C) cycling over the course of the year. These metrics of estuarine ecosystem function indicated that the estuary switched from a net sink for N during floods to a net source of N during droughts while sustaining high rates of primary production throughout the year. The estuary became a net sink for N as a result of increased denitrification rates following a storm event. Both GPP and CR increased immediately following a flood event, and the system was driven to net heterotrophy. Copano Bay became a net source of N during periods of prolonged base flow via increased N fixation rates. Internal N cycling became increasingly important during periods of base flow, including increased rates of dissimilatory nitrate reduction to ammonium. This internal N cycling sustained production during periods of drought, when GPP ≈ CR. Both CR and NEP were linked to nutrient export from the inflowing rivers. N and C cycles were tightly linked but became decoupled briefly as a result of inputs of nutrients and organic matter during the period after a major storm event.