Production-respiration relationships at different timescales within the Biosphere 2 coral reef biome

Temporal relationships between organic carbon production and respiration are not well understood in coral reef ecosystems. We monitored dissolved oxygen (O2) concentrations within the Biosphere 2 coral reef biome over a 3-yr period (1 January 1997–1 January 2000), to calculate daily rates of gross production (P), community respiration (R), and net community production (NCP). P averaged 170 mmol O2 m22 d21, and R averaged 173 mmol O2 m22 d21, which yielded a mean NCP of only 23 mmol O2 m22 d21, or a difference of only 2%. This long-term balance between P and R reflects the closed-system design of the biome. The SD of P, however, was 656 mmol O2 m22 d21; that of R was 647 mmol O2 m22 d21; and that of NCP was 626 mmol O2 m22 d21 or 615% of mean P. To better resolve the behavior of P, R, and NCP at different timescales, each of these time series were spectrally decomposed into four frequency bands that corresponded to four timescales: ,1 week, 1 week–1 month, 1–3 months (subseasonal), and .3 months (seasonal to annual). At timescales .1 month, the variance of P was not different from the variance of R. At timescales of ,1 month, the variance in P was significantly greater than the variance in R (P , 0.01). Furthermore, the correlation between changes in P and changes in R became weaker with decreasing timescale. We suggest that changes in P and R become well matched at timescales that coincide with the turnover of carbon by the dominant macroalgae within the biome (1–3 months). In addition, we conclude that measurements of NCP based on daily or weekly data sets are not good indicators of the long-term net metabolic performance of coral reefs. Coral reefs are mosaics of autotrophic and heterotrophic communities that are often arranged to maximize consumption of produced organic matter (Odum and Odum 1955; Smith and Marsh 1973; Kinsey 1979; Crossland and Barnes 1983; Barnes and Devereux 1984). Consequently, it is suggested that spatial integration of local net production should lead to average rates of gross production (P) and community respiration (R) that are more balanced at larger spatial scales, P/R ; 1.0 (Kinsey 1985; Hatcher 1988, 1990; Crossland et al. 1991; Gattuso et al. 1998). Yet P and R change seasonally, doubling from winter to summer (Atkinson and Grigg 1984; Kinsey 1985). These seasonal changes appear to be independent of latitude and are likely caused by changes in temperature and incident light (Kinsey 1985). Little is known, however, about how well changes in P and R are coupled on timescales much shorter than seasonal variations (i.e., days to weeks). Are changes in R as tightly coupled 1 Corresponding author ([email protected]). Acknowledgments We thank Heidi Barnett and Francesca Marubini for collecting data from the mesocosm and Mark Merrifield and Steve Smith for help with the analyses. This research was supported by University of Hawaii Sea Grant Program project numbers R/EL-1 and R/CR-1 as well as by the Biosphere 2 Corporation, Tucson, Arizona. This is Hawaiian Institute of Marine Biology contribution 1116 and School of Ocean and Earth Science and Technology 5805. with changes in P on short timescales (days to weeks) as they apparently are on seasonal timescales? If so, are rates of net community production (NCP) by reef communities no different from one time to the next and from one timescale to the next? In this study, to determine the temporal behavior of NCP, we analyzed temporal changes in P and R of a completely enclosed, self-sustained coral reef mesocosm at the Biosphere 2 Center. The Biosphere 2 coral reef biome is an experimental, high-latitude, low-energy coral reef community in which accurate measurements of P, R, and NCP are made. Rates of P and R are similar to those found in other high-latitude, low-energy environments (Atkinson et al. 1999). The biome environment differs from natural, high-latitude reefs in that both temperature and flow conditions are tightly controlled, whereas only incident light varies according to local weather. The reef is completely closed with respect to the biogeochemical budgets of carbon, nitrogen, phosphorus, and trace metals. We monitored dissolved O2 in this system for 3 yr, achieving a high-quality data set of daily rates of P, R, and NCP. The present study is an analysis of the relationships among P, R, and NCP within the reef mesocosm at different timescales. Materials and Methods Biosphere 2 reef biome—The Biosphere 2 (B2) Center is located ;15 km north of Tucson, Arizona in the foothills