Impact of eutrophication on the occurrence of Trichodesmium in the Cochin backwaters, the largest estuary along the west coast of India

Abstract

Phytoplankton studies in early 1970s have shown the annual dominance of diatoms and a seasonal abundance of Trichodesmium in the lower reaches of the Cochin backwaters (CBW) and adjacent coastal Arabian Sea during the pre-summer monsoon period (February to May). Surprisingly, more recent literature shows a complete absence of Trichodesmium in the CBW after 1975 even though their seasonal occurrence in the adjacent coastal Arabian Sea continued without much change. In order to understand this important ecological feature, we analyzed the long-term water quality data (1965-2005) from the lower reaches of the CBW. The analyses have shown that salinity did not undergo any major change in the lower reaches over the years and values remained >30 throughout the period. In contrast, a tremendous increase was well marked in levels of nitrate (NO(3)) and phosphate (PO(4)) in the CBW after 1975 (av. 15 and 3.5 μM, respectively) compared with the period before (av. 2 and 0.9 μM, respectively). Monthly time series data collected in 2004-2005 period from the lower reaches of the CBW and coastal Arabian Sea have clearly shown that the physical characteristics like salinity, temperature, water column stability, and transparency in both regions are very similar during the pre-summer monsoon period. In contrast, the nutrient level in the CBW is several folds higher (NO(3), 8(;) PO(4), 4; SiO(4), 10; and NH(4), 19 μM) than the adjacent coastal Arabian Sea (NO(3), 0.7; PO(4), 0.5; SiO(4), 0.9; and NH(4), 0.6 μM). The historic and fresh time series data evidences a close coupling between enriched levels of nutrients and the absence of Trichodesmium in the Cochin backwaters.

Spatial and temporal distribution in the biochemical composition of sedimentary organic matter in a tropical estuary along the west coast of India

“…Similar assumptions have been quoted [37,69]. Moreover, nutrient fortification is a frequent syndrome observed in the Cochin estuary [41,43,44] which eventually enhances algal population leading to lowering of TOC:TN values [24,34,56]. Thus, the organic matter summation in Cochin estuary is profoundly under the influence of amalgamation of marine, terrestrial and sewage inputs.…”

Interplay of diverse environmental settings and their influence on the plankton community off Myanmar during the Spring Intermonsoon

“…The long whitish bands of Trichodesmium bloom evident in Fig. 6, differed from the routine thick yellowish band reported earlier (Jyothibabu et al, 2003;Martin et al, 2013;Mohanty et al, 2010), indicated the late stage of the bloom decomposition.…”

“…It is experimentally proved that Trichodesmium can assimilate nutrients such as NO 3 , NH 4 and PO 4 (Holl and Montoya, 2005;Lin et al, 1998;Martin et al, 2013;Ohki et al, 1986;Stihl et al, 2001). Paradoxically, when the nutrients exist in high concentrations, they inhibit the growth of Trichodesmium (Holl and Montoya, 2005;Lin et al, 1998;Martin et al, 2013;Ohki et al, 1986;Stihl et al, 2001).…”

“…Paradoxically, when the nutrients exist in high concentrations, they inhibit the growth of Trichodesmium (Holl and Montoya, 2005;Lin et al, 1998;Martin et al, 2013;Ohki et al, 1986;Stihl et al, 2001). However, a few field studies observed an increase in PO 4 concentration linked with Trichodesmium blooms, and this has been attributed to the exudates from bloom biomass (Chang et al, 2000;Mohanty et al, 2010).…”