Nature and sources of suspended particulate organic matter in a tropical estuary during the monsoon and pre-monsoon: Insights from stable isotopes (δ13CPOC, δ15NTPN) and carbohydrate signature compounds

Mandovi estuary, on the west coast of India, behaves like a fresh water system during the monsoon (June to September), and as a marine system during the pre-monsoon (October to May). The effect of these contrasting conditions on the concentration and composition of suspended particulate carbohydrates was evaluated. Suspended particulate matter (SPM) was collected from 9 stations in the Mandovi estuary during the monsoon (August) and pre-monsoon (March), and was analyzed for δ13CPOC, δ15NTPN, particulate carbohydrates (PCHO), particulate uronic acid (PUA), and particulate neutral carbohydrate (PNCHO) concentration and composition. The concentrations of PCHO, PNCHO and PUA varied from 2 to 9 μMC, 2 to 7 μMC, and from 0.14 to 0.39 μMC, respectively during the monsoon; and from 4 to 10 μMC, 3 to 9 μMC, and from 0.34 to 0.90 μMC, respectively during the pre-monsoon. Both δ13CPOC and δ15NTPN were depleted during the monsoon and relatively enriched during the pre-monsoon. When expressed as % of SPM, particulate organic carbon (POC) and PCHO concentration decreased with increasing SPM content. The average PCHO yield (PCHO-C/POC%) was 13.9% ± 4.4% and ~ 10.7% ± 2.6% for the monsoon and pre-monsoon, respectively. POC and PCHO concentrations were influenced by cell numbers of bacteria and phytoplankton during the monsoon and pre-monsoon, respectively. Glucose was the most abundant constituent of PNCHO in both the monsoon and pre-monsoon. Rhamnose, fucose, ribose, galactose, arabinose, and xylose were relatively more abundant during the monsoon than the pre-monsoon. Preferential utilization of glucose results in accumulation of deoxy sugars (i.e. rhamnose plus fucose). The ratios of hexoses/pentoses and mannose/xylose and mol% glucose were relatively lower, and mol% deoxy sugars were higher during the monsoon than the pre-monsoon. Conversely, during the pre-monsoon, monosaccharide composition did not vary much and was mostly dominated by glucose. Monosaccharide abundance and ratios suggest that organic matter was subjected to extensive diagenetic alterations during the monsoon than the pre-monsoon. Carbohydrate carbon normalized to organic carbon was not useful to assess the degradation state of organic matter. POC containing 13.9% of PCHO is transported to coastal waters during the high discharge monsoon period. Dilution, differences in sources, biodegradation and physiological state of phytoplankton appear to affect the transport of particulate carbohydrates in the Mandovi estuary.

► Distribution, sources and fate of carbohydrates in a tropical estuary was investigated.
► Carbohydrate concentrations were influenced by bacteria and phytoplankton in monsoon and pre-monsoon, respectively.
► Extensive alteration in POM was observed during monsoon.
► Carbohydrate yield is not useful to assess degradation state.
► Dilution, degradation, sources and phytoplankton physiology influenced carbohydrate transport.