Diurnal and Tidal Influences on Phytoplankton Community Structure in a Tropical Mangrove Ecosystem

Teekas, Lokdeep (2016) Diurnal and Tidal Influences on Phytoplankton Community Structure in a Tropical Mangrove Ecosystem. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Lokdeep Teekas (11MS039)) Lokdeep_Thesis.pdf - Submitted Version Restricted to Repository staff only Download (1MB)

Abstract

Marine phytoplankton represent less than 1% of the Earth's photosynthetic biomass, but account for almost 45% of earth’s primary production. Although the life histories of these phytoplankton may vary with habitat and resource conditions how such high diversity of species is maintained within communities is not well understood. This is a particularly important issue as the productivity and function of marine ecosystems may be strongly dependent on the phytoplankton community. The phytoplankton communities' respone to short-term changes in resource availability is not well understood. While resources such as nitrogen, phosphorus and silica may vary primarily at seasonal scales, light resource varies dramatically over the diurnal time-scale. Being single celled organisms dependent on light for photosynthesis, phytoplankton may be far more sensitive to the diurnal change in light availability than multicellular phototrophs. Thus examining the diurnal variation in phytoplankton community structure can provide insight on the how resource availability selects for different physiological traits and life histories. Such diurnal turnover in species composition can contribute to overall species coexistence through a temporal partitioning of the resource niche. If different species and morphologies dominate at different times in the diurnal cycle a greater diversity of species can be maintained in the community. Here I test this idea by studying the diurnal variation in phytoplankton community composition and structure in a species-rich tropical mangrove ecosystem. I study community structure by characterizing the species composition, species abundance, and size structure at different time points in the diurnal cycle. From what is known about phytoplankton morphology and physiology, I hypothesize that species composition, abundance and size structure should differ substantially between night and day. In particular, I expect that smallcelled species that have regular round shapes, and therefore the smallest surface to volume ratios, should dominate under resource rich conditions while large celled species that have higher surface to volume ratios should be favored under low resource availability. Such turnover in community structure should be gradual and directional with the changes in light resource availability through the diurnal cycle.In my study I found the dependency of shape and size in resource uptake as expected. The size-dependence of resource acquisition can have a large effect on phytoplankton physiology and growth rates and thus can potentially alter phytoplankton abundance and community size structure. Experimental and theoretical evidence demonstrates that smaller cells have higher rates of nutrient uptake per unit biomass and lower half-saturation constants due to their higher surface area to volume (SA/V) ratios. The community structure is a function of nutrient availability, which in itself shows cyclic pattern, this can lead to different community structure at different time of year. Increasing the sampling effort can give us better insight at the functioning of community dynamics. My work covers only temporal variation, further studies can be done including spatial variation also.

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