Assessing the magnitude and significance of deep chlorophyll maxima of the coastal eastern indian Ocean
Faculty of Computing, Health and Science
Computing, Health and Science Faculty Office
Deep chlorophyll maxima (DCM) have the capacity to fuel substantial fractions of total water column production. The ecological importance of a ubiquitous DCM layer ranging from 50 to 120 m deep within Leeuwin Current (LC) and offshore waters of Western Australia is addressed here using data from a regional oceanographic field study conducted during the austral summer of 2000. Phytoplankton communities from surface and DCM layers were compared by examining pigments (chlorophyll a), phytoplankton carbon, photosynthetic characteristics and productivity rates estimated using 14C-based photosynthesis versus irradiance relationships. In the DCM layer, both extracted pigments (up to 0.83 mg m−3) and phytoplankton carbon (6.4–54.4 mg C m−3) were maximal, and were on average 6 and 5 times larger than in the surface layer, respectively. Sensitivity analyses were performed on production estimates using regionally relevant ranges of light attenuation (Kd=0.050–0.066 m−1) and photoinhibition (β*=0.00–0.01 mg C (mg chl a)−1 h−1 [μmol m−2 s−1]−1). These analyses provide upper and lower limits on previously reported estimates of primary production for the region, and show that small differences in light attenuation and photoinhibition can significantly affect computations of primary production and cause a shift from surface-dominated to DCM-dominated production scenarios. The contribution of the DCM layer to total water-column production ranged from a maximum of 30–70% under the scenarios examined. A regional overview of nitrate and stratification conditions in relation to the depth of the phytoplankton biomass maximum indicated that the critical balance between light and nutrients was a key factor driving DCM structure. We show that changing oceanographic conditions in both the along-shore and cross-shore directions, which included latitudinal variation in the strength of the LC, are accompanied by changes in the depth (and in turn production) of the DCM. The previously unrecognized significance of these DCM layers in the coastal eastern Indian Ocean has important implications for satellite-based estimates of production within the region.