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    Top down control of post-upwelling waters off Trivandrum: Indications from variability in DMS(P)

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    J_Coast_Environ_3_185a.pdf (8.432Mb)
    Date
    2012
    Author
    Pandey S.
    Bhonsle S.
    RituKumari
    Gauns, M.
    LokaBharathi, P.A.
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    Abstract
    Coastal upwelling is known to bring nutrient rich bottom waters to the surface. This change in chemistry triggers a series of cascading effects on the trophic levels of coastal ecosystems. Phytoplankton, besides being primary producers and carbon sink also produce a tertiary sulfonium compound dimethylsulphoniopropionate (DMSP), which breaks down mostly by bacterial mediation to a climatically relevant volatile compound dimethylsulphide (DMS). In order to probe the dynamics of DMSP in post upwelling coastal waters, we monitored variations in phytoplankton biomass, abundance and diversity along with DMSPt (total = particulate + dissolved) and DMS concentration in seawater over a period of 5 days at 6 hour intervals off Trivandrum located along the southern west coast of Indian subcontinent in November 2010. Physicochemical parameters and zooplankton abundance were also measured. Chlorophyll a averaged 1.39 mu g l sup(-1) and phaeophytin 0.49 mu g l sup(-1). Average phytoplankton abundance was 1.17E+05 l (-1) and was mainly composed of diatoms approx. 78 percent and dinoflagellates approx. 21 percent. The most abundant diatom was Pseudonitzschia spp. while dinoflagellates were represented by Gymnodinum spp. Average DMSPt concentration was 103.2 nM while DMS was 1.11nM. Highest DMSPt and DMS concentrations were in the subsurface waters at 25m depth with an average value of approx. 297nM and approx. 3nM respectively. Intriguingly, the lowest abundance of phytoplankton at 4.18E+03 l (-1) was reported at this depth with dinoflagellates comprising approx. 25 percent of the population. Intermittent peaks of DMSP coincided with the abundance of dinoflagellates in the water column. Nitrate related positively with phytoplankton (r = 0.926, p less than 0.001) suggesting its role as a nutrient for primary producers while its negative correlation with zooplankton (r = -0.576, p less than 0.001) suggests indirect control of primary producers. The top down control of phytoplankton was evident by their negative relationship with zooplankton abundance (r = -0.586, p less than 0.001). Further, the positive influence of zooplankton on DMS (r = 0.564 p less than 0.001) the negative linkages of phytoplankton with DMSPt and DMS (r = -0.863 and -0.944, p less than 0.001) corroborated our inference. Our observations thus suggest that post upwelling waters off Trivandrum is more top down controlled for the period examined.
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    http://drs.nio.org/drs/handle/2264/4881
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