The summers of 2006 and 2007 were starkly contrasting over almost all areas of Narragansett Bay in terms of water quality. July and August 2006 saw large phytoplankton blooms in the upper Bay, turbid water, and very extensive and severe (near-anoxic) low oxygen events extending over the entire upper half of Narragansett Bay. These conditions were actually worse in terms of low oxygen than the famous August 2003 fish kill event, but no large schools of menhaden were caught up in these latter events so the impacts were not visible. Summer 2007 was oddly and refreshingly different, with clear water throughout the summer, and only a minor event of low oxygen in the Providence River, with most areas not seeing hypoxia (< 3 mg/L) at all.

Researchers have noted two significant differences in the Bay for these two periods: in June 2006, the freshwater flows to the Bay hit record high levels due to intense rainstorms, likely adding significant nutrient loads from both nonpoint and point (WWTFs) as well as increasing stratification of the water column in the Bay. Extreme rainflows often negatively affect WWTF treatment, especially in systems with combined sewers. Summer 2007 (including June ’07) saw one of the driest summers on record, while the summer temperatures were not intensely record-breaking. This likely led to weaker water column stratification. Heavy June flows tend to be some of the largest flows during the summer period and are likely to have the greatest influence over both stratification and changes in nonpoint nutrient loads to the Bay during summer months (Fig. EW 6). Codiga et al. (submitted) found a strong relationship between June river flow and interannual variability of hypoxia severity, with summers associated with high June river flows tending to have more severe hypoxia in July-August of that year.
Fig. EW 6. FW flows from the Blackstone River (Woonsocket USGS gage) June- August 1929-2006. Years with flows of ~ 1200 cfs or more are identified on the graph. Data from USGS historical monthly stream gage data.

An additional difference, possibly a significant ecological shift that may be relevant to this picture, also occurred in summer 2007: a lack of the gelatinous plankton-feeding ctenophore (comb jelly), Mnemiopsis leidyi. Sullivan et al (2008) notes a shift in this species in the Bay over recent decades, changing from a late-summer resident to a near year-round population in mild winter years. This species provides strong top-down grazing pressure, eating much of the zooplankton population, including copepods and larval fish. Such grazing down of these important phytoplankton-eating herbivores could allow greater blooms of phytoplankton in the Upper Bay and Greenwich Bay, both of which have significant ctenophore populations (personal observations). The fact that the one summer in recent years with a lack of this plankton grazer turns out to have good water quality and no severe hypoxia may be coincidence or may be correlated to the lesser hypoxia conditions. Much more research needs to be done on the relationship between early summer ctenophore populations, hypoxia formation, nutrient cycling, and primary and secondary productivity of the upper Bay. An addendum to this situation is the observation that huge schools of adult menhaden remained in Narragansett Bay for almost the entire summer 2007, possibly also adding to this biological control over phytoplankton blooms since they take in the larger phytoplankton species. However, they also graze on the herbivores like copepods that tend to keep phytoplankton in check, and most summer phytoplankton in the upper Bay tend to be small dinoflagellate species not grazed on by menhaden. This has led to many researchers believing that such schools do not tend to keep phytoplankton blooms down but might actually enhance them. A possible reason they remained in the Bay so long in 2007 was the minimal interference from gelatinous ctenophores, which interfere with the menhaden’s filter-feeding behavior. Much more information on menhaden and their role in the ecosystem can be found at the RISG site based on a special menhaden symposium held in 2007.

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