In addition to what was covered in Tuesdays’ lecture, I looked further into the effects that nutrients entering the marine environment from land have on microbial communities, with regards to distributions of heterotrophic protists, and the impact of protist grazing on phytoplankton.
Heterotrophic nanoflagellates, ciliates, and heterotrophic dinoflagellates are important secondary producers in marine ecosystems. Studies have shown that on average, heterotrophic protists consume over 60% of global daily phytoplankton production.
One of the studies that I have looked at was the first to investigate heterotrophic protist community distribution and grazing impacts in the northern East China Sea (ECS), which has dynamic hydrographic conditions. The shallower western shelf water has lower salinity, higher nutrient concentrations, and higher phytoplankton stocks. These conditions are driven by the East China Sea Current (ECSC), which incorporates water diluted by Chanjiang river water (CDW). The deeper eastern section of the ECS has lower nutrient concentrations due to the year-round persistence of the Tsushima Warm Current (TWC).
The planktonic food web structure in this region can be significantly affected by the CDW, which drives distribution of biological and chemical properties in this body of water. Changes in season affect the amount of CDW present in the ESC, with inflow of river water being primarily determined by seasonal wind direction.
Five cruises were conducted over four seasons, on which environmental and biological properties were measured including the abundance and composition of heterotrophic protists, particulate organic carbon, particulate organic nitrogen, suspended solids, and chlorophyll a. Measurements and samples were taken over a range of depths and a number of locations from the water body. water temperature and salinity measurements were also taken. A dilution method measuring the total chlorophyll a concentration was used to measure specific phytoplankton growth rates and protist grazing.
Patchy distributions of biomass were observed on all cruises, with a generally higher biomass being observed in waters with higher nutrient levels and lower salinity in western areas affected by the Changjiang river discharge. These results match similar patterns where heterotophic protists are documented to have an inverse relationship with salinity. Numerically, small heterotrophic dinoflagellates were most abundant, where as the highest contribution to biomass came from ciliates
It was found that heterotrophic protists consumed a mean average of 68.2% of chlorophyll a production, and that as the phytoplankton biomass increased, grazing rates also increased. small heterotrophic dinoflagellates were most strongly correlated with phytoplankton biomass than any other type of protist. These results correlate with results obtained from similar studies elsewhere in the world.
In waters with of a low phytoplankton biomass, the biomass relationship between phytoplankton and heterotrophic protists was found to be nearly equal, showing a significant reservoir of biogenic carbon represented by heterotrophic protists.
To conclude, the results obtained from this study further back up the idea that heterotrophic protists are the major consumers of primary production in the marine environment. It also reveals that grazing of heterotrophic protists on phytoplankton is one of the most important losses affecting biomass in the northern ECS.
The influence of coastal waters on distributions of heterotrophic protists in the northern East China Sea, and the impact of protist grazing on phytoplankton
J. Plankton Res. (2012) 34(10): 886-904 first published online June 28, 2012
http://plankt.oxfordjournals.org/content/34/10/886.full.pdf+html
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