Chlorophyll

Plants are essential to Narragansett Bay’s estuarine functioning. The role played by phytoplankton (microscopic algae) and seaweeds (macro algae) is called primary productivity: the conversion of basic nutrients such as nitrogen, phosphorus, and carbon into plant tissue. This serves as the basis for the entire estuarine food web, from microscopic animals (zooplankton) to small fish such as anchovies (planktivores) to large fish, birds and mammals, including humans. Understanding primary productivity in the Bay is essential to understanding the basic ecological processes (nutrient cycling) which govern all life within it. The assessment of primary productivity is important for understanding the health of an estuary; eutrophication is, essentially, excess primary productivity. However, primary productivity is difficult to measure directly, and is therefore usually measured in terms of chlorophyll concentrations: the amount of plant chlorophyll pigments in the water column. Chlorophyll provides a rough yardstick of the abundance of primary productivity in the estuary, and excess amounts are the “fuel” for hypoxia. However, it does not fully capture changes that can take place in an estuary, such as shifts in phytoplankton species. Depending on the species involved, this can be quite important since some species make better food sources than others, and some species can be quite problematic (e.g., the “brown tide” species, Auereococcus anophagefferens).

The network of buoys that monitor hypoxia includes instrumentation that measures chlorophyll fluorescence and provides data that can be used to infer relative comparisons of phytoplankton concentration. The data are used in combination with the other data collected to identify when and where algal blooms are occurring. URI has regularly monitored phytoplankton north of Fox Island for almost 50 years, resulting in a long- term dataset of primary productivity for the lower portion of the Bay. URI also monitors chlorophyll levels at the Graduate School of Oceanography (GSO). URI monitors both the concentration of chlorophyll and changes in dominant species at the lower-Bay site. Special studies have provided a picture of the overall distribution of chlorophyll across the Bay regions.

By contrast with the URI dataset, information on types of phytoplankton and total chlorophyll levels for upper Narragansett Bay is quite limited, constituting a data gap. The fixed site monitoring at the buoy stations will help with future tracking of trends, but research that better defines past responses of the upper Bay area (e.g., sediment core work associated with past primary productivity) is critically needed for the upper half of the Bay to better assess the trends seen at the URI station mid-bay.

Status and Trends:

In Narragansett Bay, the highest levels of phytoplankton are found in surface waters of the Seekonk (during the growing season, levels can exceed 60 ug/L). Primary productivity is very high (at times ranging 20--60 ug/L) below the Fields Point WWTF in Providence, down to Gaspee Point, as well as in the western Greenwich Bay and lower Taunton River area. The Upper Bay area north of Prudence Island, and the upper reaches of the West and East Passage have moderate chlorophyll concentrations, ranging from less than 5 to approximately 20 ug/L. The URI lower Bay stations (Fox Island and GSO) areas experience much lower chlorophyll levels (less than 20 ug/L and less than 5 ug/L respectively).

 In the last 30 years, the period of the maximum annual phytoplankton bloom has decreased during the winter-spring (January-April), while summer blooms have slightly increased (June-September) and shifted to warm water diatoms in the mid to lower Bay (below Wickford), possibly due to ecological changes associated with increased water temperatures (+1 to +2° C). The overall effect is a decrease in annual chlorophyll levels at the mid-bay station. Summer bloom chlorophyll concentrations are much greater in the upper half of the Bay compared with lower Bay areas, and it is unclear if the same change has occurred in those areas.

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