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Biological oceanography

Rainwater floods an ecosystem of productivity

Citation: Varkey, D., Mazard, S., Jeffries, T.C., Hughes, D.J., Seymour, J., Paulsen, I.T., Ostrowski, M., 2018. Stormwater influences phytoplankton assemblages within the diverse, but impacted Sydney Harbour estuary. PLOS ONE 13, e0209857. https://doi.org/10.1371/journal.pone.0209857

Why phytoplankton within Sydney Harbor?

Phytoplankton are at the base of the food web and fuel the productivity of marine life. Primary productivity depends on various environmental factors such as light, nutrients, temperature and salinity. Phytoplankton community composition influences the number of trophic levels in the food web which drives the biodiversity of higher trophic level species.

The 30 sampling sites across Sydney harbor.

In this study, scientists examined the community composition of phytoplankton within Sydney Harbor, a highly urbanized estuary. Most previous studies within Sydney Harbor have examined the impact of harmful algae blooms instead of the phytoplankton community and the environmental drivers of phytoplankton diversity. Like many other regions of the world, Sydney Harbor has been impacted by anthropogenic stress from urbanization of the surrounding area. Due to the lack of freshwater inflow into the harbor, phytoplankton in the estuary rely on nutrients from storm run-off for cellular function. Understanding the factors that affect phytoplankton community composition is important for the economic services it provides, such as fish for the local population.

How did the researchers observe the phytoplankton community composition?

In this study, scientists collected water from 30 sites within Sydney Harbor across six regions of the estuary. Sample collection occurred twice during one year—once during a high rainfall period and once during a low rainfall period. Phytoplankton are tiny organisms, many of which cannot be seen with the naked eye. Therefore, they need to be observed with a microscope.

A community of diatoms typically found in a coastal estuary. Photo courtesy of the Narragansett Bay Plankton Time Series.

However, examining phytoplankton diversity with a light microscope is difficult due to very small visual differences between species. To differentiate between species across sampling sites, the authors used various molecular techniques. Just like you and me, phytoplankton have DNA that encodes various functions of the cell. So the authors collected DNA from the water samples and analyzed the sequences to differentiate phytoplankton species within Sydney Harbor. In order to assign species to the DNA sequences, the authors used bioinformatics, a method that enables the analysis of thousands of DNA sequences using various computer software programs. Using DNA analyses instead of light microscopy is a much more efficient method for evaluating community composition because it can help the researcher differentiate between very similar species.

What did the researchers find?

Phytoplankton are autotrophs meaning they photosynthesize to produce their own energy. There are prokaryotic phytoplankton and eukaryotic phytoplankton which are distinguished by the absence or presence of membrane-bound organelles. Most of the cyanobacteria observed in Sydney Harbor were Synechococcus. Interestingly different clades, or groups, of Synechococcus were found during the two types of rainfall events (clade II was abundant during high rainfall events and clade I was abundant during the low rainfall events). For the eukaryotes, diatoms dominated during high rainfall events whereas dinoflagellates dominated during low rainfall periods. Diatoms are a highly-productive functional group so they support a high-quality food-web. An abundance of diatoms typically leads to greater total biomass at higher trophic level organisms, such as commercially important fish species. Therefore, diatoms are essential for estuarine ecosystem functioning.

Intense rain over Sydney Harbor. Climate change models suggest a decrease in the frequency of these storms but an increase in storm intensity, which will affect phytoplankton community composition.

Why does this matter?

Previous studies within Sydney Harbor have considered how stormwater inflow into the harbor influences the microbial community in a negative way (such as increased fecal coliforms that have ramifications for human health). However, this study focused on the primary producers of the estuary and found that there is a diverse and healthy phytoplankton community that exists during high rainfall events. Sydney Harbor has limited nutrient enrichment from riverine input so periodic nutrient inflow from rain events (stormwater) may be essential for primary productivity of the estuary. The findings of this study highlight the importance of nutrient inputs from rain events for phytoplankton productivity, especially during the recent drought conditions that Sydney has experienced. With precipitation rates likely to shift with climate change, it is essential to understand how the base of the marine food-web will be affected to help predict future changes to overall ecosystem health.



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