Spatio-temporal variation of optically active substances in the coastal waters off Orissa from Rushikulya to Dhamra (east coast of India)

Surface concentration of gelbstoff and vertical structure of chlorophyll-a (chl-a) and pheophytin-a were determined in coastal waters off Orissa along five transects. Depth-dependent concentrations of NO3- and PO43- at several stations along the Paradip transect (Tr.), Tr.-IV were also studied. The concentration of chl-a differed greatly between various sampling stations. Along a transect the concentration of the pigment was mostly higher in the near-shore waters (station depth less than or equal to 20 m) than in the offshore waters (station depth > 20 m). And irrespective of the depth of the sampling stations and the transects, the pigment was more concentrated towards the bottom than towards the surface (0 in) and sub-surface (10/15 m) layers of the water column. Similar to chl-a both NO3- and PO43- were found to be more concentrated towards the bottom than towards the surface of the water column, but none of them really regulated the growth of phytoplankton. Pheophytin-a constituted mostly less than 5-7 % of the total chl-a value in the surface waters, and the concentration of gelbstoff differed greatly in time and space. Secchi-disc transparencies indicated one attenuation depth (for light) of around 4 to and 7 in for the near-shore and the offshore waters, respectively, suggesting the use of only the surface chl-a value for validation of the satellite derived values for the pigment. The study also indicated that chl-a retrieval from the satellite image need not be corrected for the presence of pheophytin-a. because of its low presence in the surface waters, but must be corrected for interference by the presence of high concentration of the gelbstoff.