Seasonal shift in net ecosystem production in a tropical estuary

Net ecosystem production was examined in the Mandovi and Zuari estuaries (southwestern India) and the adjoining coastal waters for a period of 1 yr (January to December 1998). The study period encompassed premonsoon, monsoon, and postmonsoon seasons. At the estuarine stations, net ecosystem production showed monthly variation and a transition from net autotrophy of 49 mmol C m22 d21 during the nonmonsoon seasons (premonsoon and postmonsoon) to net heterotrophy of 246 mmol C m22 d21 in the monsoon season. Seasonal monsoon-driven changes such as increased allochthonous inputs resulted in enhanced heterotrophic respiration and reduced primary production in the estuaries. In the coastal station, the monthly variation in net ecosystem production was not significant, and net heterotrophy was prevalent whenever measurements were made, thereby potentially serving as the net source of carbon dioxide to the atmosphere. Results suggest that the excess organic matter from these tropical estuaries supports heterotrophy in the adjacent coastal ecosystem. The overall metabolic balance in any aquatic system depends on the sum of two fundamental and complementary variables: primary production (P) and community respiration (R). Measurements of these variables are a prerequisite to assess the trophic status of aquatic ecosystems. Del Giorgio et al. (1997) suggested that metabolic balance depends on the primary productivity of the system. The rate of plankton growth efficiency and respiration are important factors that determine the fate of primary production. Recently, heterotrophy has been reported from euphotic layers of the subtropical Northeast Atlantic (Duarte et al. 2001; Hoppe et al. 2002), and heterotrophy is not restricted to oligotrophic systems. Of late, there has been considerable debate on the role of planktonic communities as sources (del Giorgio et al. 1997; Duarte et al. 2001) or sinks (Williams 1998) of carbon in subtropical and temperate waters. The coastal ecosystems, especially tidally influenced estuaries, have drawn much attention, since they are the most geochemically and biologically active areas. They receive inputs of terrestrial organic matter and nutrients through riverine runoff, which establishes a variable nutrient gradient in time and space. There is even a debate on whether coastal systems are nominally heterotrophic or autotrophic on a net annual basis (Smith et al. 1991). We hypothesize that nutrient load during the monsoon would increase autotrophy and excess organic matter would be transported to the coastal 1 To whom correspondence should be addressed. Present address: Shanta Nair (Shanta Achuthankutty), Microbiology Laboratory, Biological Oceanography Division, National Institute of Oceanography, Dona Paula, Goa 403 004, India ([email protected]). Acknowledgments We thank D. L. Kirchman, University of Delaware, for his valuable comments on the previous version of this manuscript and Dileep Kumar, NIO, for helping us with the flux calculations. We highly appreciate the two anonymous reviewers for their valuable comments and suggestions, which have improved the quality of this manuscript. The Office of Naval Research (USA) under the IndoU.S. collaborative project on Trophic Dynamics supported this research. A.S.P. is supported by a Senior Research Fellowship grant from the Council of Scientific and Industrial Research (CSIR), New Delhi. NIO contribution 3818. waters. Thus there may be a temporary shift in status (net autotrophic to net heterotrophic) of the coastal ecosystem. To test this hypothesis, the present study was carried out in the Mandovi–Zuari estuarine system, which has been considered productive based on its primary production (Devassy and Goes 1989). This study area, which forms a part of the Northern Indian Ocean (southeastern Arabian Sea), is characterized by extreme wind forcing and seasonal reversal monsoon winds, which is an annual recurring phenomenon in these estuaries. This region experiences a spell of heavy precipitation of the order of 250–300 cm yr21, much of it occurring during the southwest monsoon (June–September). During the dry season (November–May), the total precipitation drops to less than 10 cm (Shetye and Murty 1987). Large amounts of materials are transported through these estuaries following heavy rains during the monsoon period. This extent of input of organic matter might have a significant effect on the metabolism of the system. In the present study, the following questions were addressed. Is this tropical estuarine system autotrophic and subsidizing adjacent coastal waters? Or, is it heterotrophic and consequently dependent on allochthonous organic inputs from surrounding areas? Are inputs consumed by the resident heterotrophs, thus supporting their growth and respiration, or are they exported to the adjacent coastal waters? Does this input govern the trophic status of the system? The importance of organic matter outwelling from Mandovi–Zuari estuaries to the adjacent coastal waters was assessed by measuring primary production and community respiration. In this study, we also examined whether there was a seasonal change in the trophic status in tropical estuaries and the adjacent coastal waters of the Arabian Sea. Assessment of the relative contributions of these estuaries to largescale carbon budgets could lead to a better understanding of the role of tropical estuaries in general.