Fig. EW 5. Dissolved Oxygen Concentrations Aug. 15, 2001 and line showing shallow apparent Redox Potential Discontinuity (RPD) in Narragansett Bay, indicating poor benthic habitat (based on Vallente et al. 1992). Oxygen values are from D. Murray, Brown University.
Based on synoptic surveys (Prell et al. 2006, Saarman et al. 2008) and fixed site continuous monitoring (RIDEM 2008), hypoxic events are concentrated mainly around July and August (Saarman et al. 2008). The most severe hypoxia, both in terms of intensity (lowest levels reached) and duration, follows the N-S pollution gradient the Bay (Fig EW5), with lowest oxygen levels (1-2 mg/L) extending over a period of days to weeks in the Seekonk and Providence tidal rivers. The severity of hypoxia generally decreases as one heads down-bay. However, western Greenwich Bay appears to act unique in experiencing short but extremely severe hypoxic events, and the upper West Passage (Mount View area) shows less frequent but still severe bouts of hypoxia. The Fox Island long-term monitoring station halfway down the Bay does not represent the oxygen conditions further up the Bay, but even here there have been some recent (summer 2006) severe low oxygen readings (< 2 mg/L) documented (Saarman et al. 2008).

Saarman et al. (2008) have shown high internal correlation in dissolved oxygen levels for four areas of the upper half of the Bay: 1) the Providence River, Upper Bay and ship channel ; 2) Greenwich bay ; 3) upper West Passage ; and 4) Mount Hope Bay. These areas have strong internal spatial coherence and could be monitored with critically-placed “sentinel” buoys to represent wider Bay areas.
Click here to view the available data

Frequency and Duration of Hypoxia

Typical hypoxic conditions differ depending on where you are in the Bay. The most severe conditions are in the Seekonk and Providence Rivers, especially just below the WWTF effluent areas, and western Greenwich Bay (Fig EW 5) Based on data from continuous monitoring stations, the duration of hypoxia in the Providence River and Conimicut Point ranges from 2 days to ~ 1 week for typical events, although shorter hypoxic periods occur, and there are occasional rare extended events ( > 2 week). Such extended events tend to also be of fairly severe intensity (< 1 mg/L)(Codiga et al. submitted).

The area just north of Prudence Island experiences events of 1 day to ~ 1week, with rare extensions beyond 1 week. Limited (2006) data from Mount View and Quonset Point indicate that the upper West Passage sometimes exhibits severe hypoxic events that can last 1–10 days (Codiga et al., submitted).

In reality, the western side of Greenwich Bay has the most severe hypoxic impacts in terms of severity (< 1 mg/L) and frequency (up to 10-12/summer) compared with any other area of the Bay measured to-date. However, these low oxygen Greenwich Bay events are usually short-lived (1-3 days) , with rare extensions of >1 week. This is not necessarily because of greatest nutrient loadings (although they are quite high for this shallow embayment). Rather, this is likely due to poor circulation in the western side of Greenwich Bay, which maintains poor water quality and does not promote flushing with cleaner Bay waters (see below section).. The short duration of events is likely due to tidal and wind mixing events that can rapidly mix waters in this fairly shallow embayment. The high frequency of hypoxic events is likely a reflection of both the biological response to the local nutrient load and the poor circulation in the western side, where hypoxia is concentrated. It can break up easier than other areas of the Bay, but is more susceptible to brief bouts as soon as environmental conditions become conducive to development of hypoxia. In short, Greenwich Bay, like many poorly flushed areas, is highly sensitive to nutrient loads, and will respond rapidly to additions from all sources.

Greenwich Bay

As noted above, Greenwich Bay does not follow the down-bay impact gradient. The worst water quality conditions are concentrated on the western side (Fig EW 5, Shumchenia 2008). The major nutrient loads are estimated to be dominated by input from the Upper Bay entering through the mouth of Greenwich Bay at Warwick Neck. However, the western side appears to be poorly flushed based on preliminary modeling (Abdelrhmann 2005, Kincaid, personal communication 2007). In addition, groundwater contributions carrying nitrogen from the high density of septic systems in the area are a significant component of loading to this embayment (CRMC 2005, Nowicki and Gold 2008). Accurate estimates of groundwater contribution and movement rates are significant data gaps for understanding the improvement response time of the Greenwich Bay system to sewering. However, better data would not change management options and responses. The present efforts under the RICRMC Greenwich Bay Special Area Management Plan (SAMP) to sewer most of this area is still the most reasonable response to a groundwater source. The greatest change would be if areas not presently planned for sewering in the Greenwich Bay SAMP turn out to be major loading areas. It is ironic that this area was once one of the best high quality habitat areas of the Bay, being known for extensive eelgrass beds and significant scallop resources (Doherty 1995). It is probably the very aspect of poor flushing that allowed this (and other small embayments) to allow retention of high densities of planktonic (floating) larval species for recruitment as juveniles and adults. This is a warning to managers that poorly flushed, high density benthic resource areas are likely to be some of the most vulnerable areas to nutrient additions (Deacutis 2008). Such areas should be provided with conservative nutrient management protection controls.

Mount Hope Bay and Sakonnet River – Data Gaps

Mount Hope Bay experiences mild hypoxia (< 4.0 mg/L), but it seems to rarely go below 3.0 mg/L based on studies by the UMASS School for Marine Science and Technology (SMAST) and the recent deployment of a Mount Hope Bay mid-bay station within the “fixed site” RIDEM monitoring effort. Only the mouth of the Lee and Cole Rivers and the lower Taunton River mouth and the Taunton River tidal area above Fall River exhibit significant concentrations of nutrients and chlorophyll, as well as poor oxygen levels (< 3 mg/L) (Howes and Samimy 2007, Krahforst and Carullo 2008). s. Excess nutrient impacts were identified as one of the stressors for the 1960s in an old publication for the Taunton River and Mount Hope Bay (MASS Div Mar. Fish 1974 Monograph, cited in Krahforst and Carullo 2008). It is unclear why larger areas of Mount Hope Bay are not presently experiencing larger chlorophyll bloom and hypoxia responses based on the large freshwater flow and nutrient loading from the Taunton River , as well as a significant source from the Fall River WWTF along the SW shore. It may be due to nutrient uptake within the lengthy tidal reach of the Taunton River, the shallow nature of Mount Hope Bay itself (< 6m depth), or it may be due to other unknown factors. This is a significant data gap, and more work to examine primary productivity responses to the nutrient loads in Mount Hope Bay is clearly needed.

Mount Hope Bay also experiences the highest temperatures of the Bay for both winter and summer periods due to the heated discharge of the largest fossil-fuel power plant in the northeast at Brayton Point (Chen et al. 2008, Krahforst and Carullo 2008, Mustard et al. 2001). The Brayton Point Power station was shown to be impacting commercially-important fish larval populations during a permit reissuance process. The present plant uses a once-through cooling system which can withdraw over 950 million gals/day, entraining and impinging planktonic organisms during the process. Following permit impact analyses, new permit requirements were issued by the U.S.EPA in October 2003. The new permit requires a change in the cooling system to a “closed loop” system that uses 2 cooling towers and reduces the withdrawal to 56 million gals/day, a 94% decrease and a 96% decrease in excess heat discharge. with . The change is estimated to eliminate 94% of the projected present fishery losses (U.S. EPA 2003). Following an appeal process, the plant owner agreed in 2007 to proceed with installation of the cooling towers. US EPA estimates the new system may be functional around 2012. Click here to view permit documentation.

The level of nuisance seaweed growth is unknown in Mount Hope Bay This is a significant data gap, and a multi-institutional (e.g. Roger Williams/Brown/URI) or bi-state effort to further investigate the ecology of this area of the Bay would help to better assess this area and better understand responses there. A single benthic community station has been established by URI graduate students (C. Calibretta, 2007. URI personal communication), but this site is close to the Brayton Power Plant and is likely affected by multiple stressors, including temperature and possibly impacts from coal dust. Much more investigation of the Mount Hope Bay system is needed to better understand the responses of this subembayment to the multiple stressors. A recent study by U Mass Dartmouth (SMAST) for the Estuary project of the MADEP provides some limited new information on nutrient loads (Howes and Samimy 2007), but matching funds to do an intensive study of Mount Hope Bay are presently lacking (MADEP personal communication).

Another data gap exists for the Sakonnet River system, which has little to no data available on nutrients, chlorophyll, dissolved oxygen or macroalgae. There are limited data on fish from two RIDEM Fish and Wildlife trawl stations, but the northernmost station has been abandoned due to interference by excess seaweeds on the bottom, strongly suggesting that the northern end at least may be experiencing problems with excess nutrients.

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