Aquatic metabolism in the Everglades: dominance of water column heterotrophy.

Using high-frequency measurements of free water dissolved oxygen (O2), we assessed gross primary production (GPP), respiration (R), and net aquatic production (NAP) in the shallow-water Everglades peatland between 1996 and 2005. We distinguish NAP from net ecosystem production since the boundary for shallow aquatic ecosystems may include aboveground GPP of wetland biota. Metabolism was estimated for 68 sites distributed among nine habitats and yielded 1085 5-d deployments or 5425 site-days. Habitats differed in vegetation composition, trophic status, and hydrology. Systemwide O2 averaged 3.8 6 2.2 mg L21 (mean 6 SD), or 49% 6 30% of atmospheric saturation. GPP, R, and NAP averaged 103 6 76, 220 6 79, and 2117 6 65 mmol O2 m22 d21, respectively. Metabolism was greater during the summer-wet season when greater irradiance, temperature, and material flux stimulate primary production and decomposition. Paradoxically, GPP was inversely related to total phosphorus (TP), with oligotrophic (TP , 7 m gL 21) open-water habitats dominated by periphyton having the highest and eutrophic (TP . 35 m gL 21) habitats with dense emergent macrophytes the lowest rates. R was greatest for moderately enriched (TP 5 15 m gL 21) open-water habitats with floating macrophytes. The prevalence of net heterotrophy, 96% of the 1085 NAP estimates, reveals the importance of aboveground biota in regulating aquatic metabolism and O2 dynamics in shallow ecosystems. R is not only regulated by the influx of aboveground autochthonous carbon but also by aquatic GPP. Carbon turnover is greater in habitats where O2 production by aquatic vegetation enables aerobic respiration. Conversely, water-column GPP is suppressed by dense emergent macrophytes, which limits O2 availability, favors anaerobic respiration, and reduces carbon turnover.