Selectivity of subtidal benthic invertebrate communities for local microalgal production in an estuarine mangrove ecosystem during the post-monsoon period

Abstract

Stable isotope analysis was used as a tool to assess the main carbon sources sustaining the benthic invertebrate communities in an estuarine mangrove ecosystem along the southeast coast of India during the post-monsoon season. In particular, we wanted to test whether the large amounts of terrestrial carbon brought in during the monsoon influence the benthic foodweb in this area, by comparing with earlier data on the pre-monsoon period. The δ 13C of the dissolved inorganic carbon (DIC) pool was spatially variable, with lower values in the mangrove creeks (−10.6 to −8.9‰) compared to those in the adjacent bay region (−4.3 to −2.6‰). Fixation of the 13C-depleted DIC in the mangrove creeks should therefore result in a partial overlap in the δ 13C signature of mangrove-derived carbon and local phytoplankton. The lack of correlation between δ 13C values of benthic invertebrates (which showed a large spatial gradient of ∼8‰) and those of sediments or suspended matter (both showing only small spatial gradient of <2.5‰) indicates that invertebrates were highly selective for locally produced algal food sources. These results are similar to those obtained during the pre-monsoon period in the same area, although in each region δ 13C values were consistently more negative (by 1–3‰) during the post-monsoon period, consistent with the seasonality in δ 13C DIC. By defining selectivity as the relative spatial gradient in consumer δ 13C compared to the δ 13C of bulk particulate organic carbon (POC) and δ 13C DIC (as a proxy for the variations expected in local producers), and assuming that the selectivity is similar along the salinity gradient, we estimate that benthic invertebrates rely almost entirely on locally produced microalgal carbon sources. A critical evaluation of earlier studies shows that there is currently no unambiguous evidence for a trophic role of mangrove litter in sustaining subtidal benthic and pelagic invertebrate communities in adjacent aquatic systems.