Chowdhury Mintu, Biswas Haimanti
CSIR National Institute of Oceanography, Biological Oceanography Division, Dona Paula, Goa 403004, India.
CSIR National Institute of Oceanography, Biological Oceanography Division, Dona Paula, Goa 403004, India.
Sci Total Environ. 2023 Jun 20;878:162963. doi: 10.1016/j.scitotenv.2023.162963. Epub 2023 Mar 21.
The western Indian continental shelf is characterized by contrasting biogeochemical features from south to north mostly governed by monsoon wind forcing. Here we present the first detailed study on the phytoplankton community (analyzed by marker pigments and microscopy) during the summer monsoon addressing the interannual variability (Aug 2017 and 2018) along the Western Indian Continental Shelf waters (8° N-21° N, at 200 m isobaths). A distinct interannual variability in monsoon impacted the hydrography and nutrient stoichiometry that was further cascaded to the phytoplankton community structure. The timing of the monsoon onset, wind speeds, and the strength of the alongshore wind component were the major factors that governed the interannual variability in the development and progression of the upwelling as well as phytoplankton bloom. The upwelling-dominated southwestern shelf was characterized by cold, nutrient-rich waters dominated by the marker pigment of diatoms, fucoxanthin, and microscopy also revealed the presence of large, chain-forming diatoms (Chaetoceros sp.; Dactyliosolen fragilissimus). In contrast, low nutrient warm waters in the northern shelf housed nanophytoplankton and picophytoplankton along with small diatoms (Thalassiosira sp.) and dinoflagellates (Gymnodinium sp.). Interestingly, in a few stations in the southern shelf, the surface waters were low saline that was intensified in 2018 compared to 2017 inhibiting upwelled water from reaching the surface. Consequently, due to low nutrient levels caused by reduced upwelling and low salinity, picoplanktonic cyanobacteria dominated replacing the larger diatoms that were found plenty in 2017. Likely, such a niche shift adversely impacted the planktivorous fish catch (Indian Oil Sardine) that was seen to be less in 2018 relative to 2017. Any further changes in monsoon variability and freshwater flow to this region may have direct consequences on the phytoplankton community as well as fisheries.
印度西部大陆架的特点是从南到北具有截然不同的生物地球化学特征，这主要受季风风力的影响。在此，我们展示了关于夏季风期间浮游植物群落的首次详细研究（通过标志性色素和显微镜分析），该研究针对印度西部大陆架水域（北纬8°至21°，等深线200米处）的年际变化（2017年8月和2018年）。季风的明显年际变化影响了水文状况和营养化学计量，进而进一步影响了浮游植物群落结构。季风开始的时间、风速以及沿岸风分量的强度是控制上升流发展和推进以及浮游植物大量繁殖年际变化的主要因素。以上升流为主的西南大陆架的特点是海水寒冷、营养丰富，以硅藻的标志性色素岩藻黄质为主，显微镜观察还发现了大型的、链状形成的硅藻（角毛藻属；脆弱根管藻）。相比之下，北部大陆架的低营养温暖水域中存在微型浮游植物和微微型浮游植物，以及小型硅藻（海链藻属）和甲藻（裸甲藻属））。有趣的是，在南部大陆架的一些站点，表层海水盐度较低，与2017年相比，2018年这种情况更加严重，抑制了上升流海水到达表层。因此，由于上升流减少和低盐度导致营养水平较低，微微型浮游蓝细菌占主导地位，取代了2017年大量存在的较大型硅藻。很可能，这种生态位转移对浮游生物食性鱼类的捕捞产生了不利影响，2018年相对于2017年，印度油沙丁鱼的捕捞量减少。季风变化和该地区淡水流量的任何进一步变化可能会对浮游植物群落以及渔业产生直接影响。
