Eutrophication: Excessive Nutrient Load

Primary Production in the Water Column:

Phytoplankton

As noted in the page on eutrophication, one of the first responses to high levels of nutrients, especially nitrogen, in marine waters is a proliferation of microscopic plant life, phytoplankton. For most of Narragansett Bay, nitrogen is the dominant nutrient controlling such plant growth. However, in the Providence and Seekonk Rivers, phosphorous may also play a role (Granger 1994). The sources of nitrogen and phosphorous have been well documented for Narragansett Bay (Nixon et al 1995, Nixon et al, 2005, Nixon et al 2008, Pryor 2004, Pryor et al 2007).

The greatest source (> 60%) comes from treated sewage effluents discharging directly into the Providence/ Seekonk River or upstream along the Blackstone and Pawtuxet Rivers, as well as the Taunton River and southeastern shore for Mount Hope Bay. Stormwater runoff likely contributes a majority of the remainder, with atmospheric deposition some component of this (Nixon et al, 2005, Nixon et al 2008, Pryor et al 2007).

Highest levels of phytoplankton are found in the lower Providence River, where nutrient ratios favor flagellates over diatoms. Chlorophyll maximum and highest primary production rates occur below the major WWTF at Fields Point from Gaspee Point into the Upper Bay area (north of Prudence Island), with inner western Greenwich Bay (west of Sally Rock) coming in a close second based on fixed site monitoring data (Deacutis 2008, Oviatt 2008, Oviatt et al 2002, Saarman et al 2008, Smayda and Borkman, 2008). High levels of chlorophyll also exist in the lower Taunton River (Krahforst and Carullo 2008 ). In addition, the summer bloom chlorophyll concentrations are much greater than the Winter-Spring blooms in this area compared with the lower Bay. Winter-Spring blooms tend to be more even in chlorophyll levels across all regions of the Bay (Smayda and Borkman, 2008). Smayda and Borkman (2008) noted that maximum chlorophyll concentrations did not occur at the area of highest nutrients but at an intermediate concentration in the lower Providence River. The phytoplankton do not fully utilize all the nutrients above Fields Point (Oviatt 2008, Smayda and Borkman 2008). The reason for this is not known, but researchers have provided several hypotheses: short residence time (washout) from the upper Providence River; growth inhibition by excessively high nutrients; or response to unknown water quality problems. Washout was considered the most likely scenario (Smayda and Borkman 2008), but this will be much less applicable to summer dry low-flow conditions. One possibility not mentioned might be impacts due to the higher level of chlorine residual that once occurred outside major WWTFs such as Fields Point. The Smayda surveys took place between 1985-1987. Dechlorination at major WWTFs was not initiated until ~ 1997 (Table EW1). As more chlorophyll data is acquired, it would be useful to compare the present gradient with Smayda’s earlier hyperbolic gradient. If chlorine was inhibitory, the decrease should no longer be obvious, while if washout is the cause, this lower phytoplankton above Fields Point should still exist.

The shallow Seekonk River (avg. 1.3m, Chinman and Nixon 1985) has very little published data, but what is available (NBC unpublished data 2005-07) suggests that it too may experience occasional high phytoplankton productivity levels (Deacutis, 2008). However, this estuarine area has both a lower salinity range and low retention time, which may sometimes restrict phytoplankton levels. We noted unusually high levels of benthic primary productivity (benthic diatom mats of Melosira sp.) based on macroalgal surveys by the NBEP. It is recommended that the Seekonk be included in any primary productivity work done in the upper Bay to better define this mesohaline shallow zone.

In the last 30 years, the maximum bloom period has shifted from the winter-spring bloom period to summer , possibly due to increased water temperatures (Oviatt et al. 2002, Smayda and Borkman 2008, Sullivan et al. 2008). These summer blooms often involve small dinoflagellates as well as the larger, more nutritious diatom species found in the winter-spring blooms. In addition, the data indicate that the overall annual average concentration of chlorophyll in the lower Bay has decreased due to this loss of the winter-spring bloom, resulting in increasing water clarity at the lower Bay Fox Island station (Smayda and Borkman 2008). There are some recent research developments related to absolute chlorophyll level measurements. Many programs, including the URI long term chlorophyll monitoring Fox Is. site, freeze chlorophyll filter samples prior to analysis using the spectrophotometric method for chlorophyll. However, recent research (Graff and Rynearson 2008) points to a loss estimated at up to half of the actual chlorophyll concentration based on non-frozen samples. Therefore, interpretation of chlorophyll concentrations needs to recognize the potential low estimate bias if based on frozen samples. The decreased trend is likely real because the freezing of samples occurred throughout the monitoring period, allowing for reasonable relative comparisons (T. Rynearson, URI, personal communication 2008).
Table EW1. Timeline for Changes in Treatment at Major Wastewater Treatment Facilities Contributing Nutrients and other pollutants to Narragansett Bay. Information compiled from RIDEM sources.

As noted by some researchers, the data suggest that nutrients are being very efficiently utilized by the phytoplankton in the lower Providence River/Upper Bay area, with a mid-Bay transition/ depuration zone of rapidly decreasing nutrients, and a lower Bay zone of lower phytoplankton productivity and nutrient levels more influenced by recycled nutrients recycled within the system and nutrients from incoming Block Island Sound shelf water than upper Bay sources (Smayda and Borkman 2008). An additional complexing issue involves changes in grazing pressure. Population growth of the ctenophore Mnemiopsis leidyi has been occurring much earlier in the summer season since at least the 1990s due to warmer Bay waters, leading to loss of phytoplankton grazers like copepods and release of phytoplankton from grazing pressure (Smayda and Borkman 2008, Sullivan et al. 2008). Logically, this increased grazing on secondary production by gelatinous zooplankton should also have repercussions on fish larval populations and benthic-pelagic coupling during periods of high ctenophore densities.

It has been noted that the strong gradient in both nutrient levels and chlorophyll concentrations means that the Bay has zones which require separate characterization studies, and one cannot extrapolate information from the lower Bay (e.g., Fox Island URI long-term monitoring station) to assess conditions in the Providence River or Upper Bay or restricted coves and harbors in the Bay (Melrose and Berman 2007, Smayda and Borkman 2008, Saarman et al 2008). It has been suggested that the Bay be broken up into an “Enrichment Zone” of the Seekonk/upper Providence River areas where the major nutrient loads enter the Bay, a “Depuration Zone” of the lower Providence River and Upper Bay area where the nutrients are taken up and chlorophyll is maximum, and an “Advection Zone” below this area of the two Bay Passages (Smayda and Borkman 2008). The Bay is broken up into a larger number of zones in Deacutis (2008) which could probably be merged into 4 zones: those areas exhibiting the most severe hypoxia-eutrophication impacts (poorly-flushed areas like western Greenwich Bay and high nutrient load areas of the Providence–Seekonk Rivers), the Upper Bay area north of Prudence Island, the upper Passages where eutrophication impacts are highly intermittent, and the areas from Northern Jamestown south past the bridges to the mouth of the Bay where eutrophication impacts are minimal.

More emphasis needs to be placed on monitoring that recognizes this Bay zonation. Critical data gaps are present for long-term monitoring data for chlorophyll in the upper zones. Costa-Pierce and Desbonnet (2008) suggest an integration of this zonation approach for an ecosystem-based nutrient management effort. As they note, more work needs to be completed for modeling of the water dynamics in the Bay at an adequate resolution to cover these zones as separate entities, and to add ecological components to these models. We are still at an early stage in these efforts, and research and monitoring that address this gap should be highly encouraged.

1st Signs of Problems –

Historical Loss of Eelgrass Meadows

The distribution of eelgrass (Zostera marina) in Narragansett Bay is strongly controlled by the clarity of the water column. However, eelgrass declined here as elsewhere along the Atlantic coast in the 1930’s due to a slime mold, Labryinthula zosterae (Short et al. 1987). Beds were re-established in many areas of the Bay by the 1940’s based on work completed by the Narragansett Bay Estuary Program (NBEP) using interview techniques with retired scallopers. A second collapse seems to have occurred around the late 1940s and early 1950s, when the scallopers noted eelgrass beds disappearing again (Fig. EW. 4,.Doherty 1995). High nutrient and chlorophyll levels in conjunction with warmer water produce strong negative impacts on this species, limiting the habitat presently suitable for growth of this important estuarine habitat. All significant eelgrass beds are now found only in the lower half of the Bay, mainly from the southern end of Prudence Island to the mouth of the Bay Passages. Areas like Greenwich Bay (“Scalloptown” in the 1900’s) which held large expanses of eelgrass meadows even in the 1940’s can no longer accommodate such sensitive habitat-forming species, likely due to the high level of nutrients in these areas, leading to loss of this important juvenile nursery habitat (Deacutis 2008).

Fig. EW 4. Historical Eelgrass beds in Narragansett Bay based on historic geodetic surveys, national herbarium collection notes, and interviews with former scallopers (Doherty 1995 and CRMC eelgrass map coverage 2007
A 1996 aerial survey developed by the NBEP and Save the Bay found only about 100 acres left in the lower half of Narragansett Bay. A more recent (July 2006) overflight study (Bradley et al. 2007) shows an increase to approximately 300 acres within the Bay, with the increase confined to the lower Bay. Part of this increase is due to more intensive ground-truthing and more accurate diver-based bed edge techniques used in the latest results. However, a re-examination of both photo sets for several specific bed areas indicates that some aspect of this increased acreage is a real increase in coverage. Because these two surveys are the first attempts to measure eelgrass habitat, they may be reflecting year to year variability. There is a clear need for annual surveys to provide a better measure of interannual variability and provide data for actual trend analyses. Present available data is inadequate for trend analyses.

Nuisance Seaweeds

Macroalgae, recognized by most citizens as seaweeds found along the intertidal shores and shallow coves of Narragansett Bay, are an important component of a healthy Bay ecosystem. They provide both food and vital habitat/ shelter to many marine animal species in the Bay. However, when nutrients become excessive, certain drift seaweeds, sometimes called “nuisance” seaweeds, begin an explosive growth phase in late summer months, often coating shallow areas with a carpet of sometimes rotting seaweed (Valiela et al. 1997). These are often “green tide” seaweeds (phylum Chlorophyta), with the most common culprit being Ulva sp., known as “sea lettuce” , although other species can and do occur in significant amounts. The complaints of foul odors due to hydrogen sulfide coming from bacterial decay at low tide in the Providence River are often linked to carpets of this species of algae rotting in extreme hot summer temperatures north of Conimicut Point.

In 2006, NBEP scientists were aided by the Rhode Island Department of Environmental Management (RIDEM) to utilize the state helicopter to take high-resolution oblique photos of the intertidal shoreline over the summer months of 2006, 2007 and 2008. The NBEP is collaborating with Dr. Giancarlo Cicchetti of the United States Environmental Protection Agency , Atlantic Ecology Division (USEPA-AED) laboratory in Narragansett, RI, conducting summer monthly low tide surveys of the Providence/ Seekonk Rivers and the western shore of Narragansett Bay as a way of tracking the dominant macroalgal growth. GIS maps of macroalgae density classes are being developed from this data. This database will provide a useful baseline to track changes to nuisance macroalgae populations in the future as nutrient levels are better controlled.

An unexpected result of this survey was the discovery that, despite high nutrient levels, extensive amounts of nuisance seaweeds are not found in the Seekonk River. Although small amounts of Ulva sp. were observed, the dominant summertime benthic primary production seems to be composed mainly of a benthic diatom mat. The shallows of this entire tidal river are often completely coated with this brownish-colored benthic diatom mat attached to sediments, rocks, and even terrestrial vegetation debris (preliminary identification as Melosira nummuloides). This is probably due to large swings in salinity and possible washout due to stormwater flow from the Blackstone River system. Based on the helicopter surveys as well as quantitative biomass surveys by Dr. Carol Thornber of URI (personal communication 2008), nuisance seaweeds often blanket the intertidal areas of shallow coves especially in the mid and lower Providence River, with maximal levels usually occurring in August. Significant amounts can also be found in the shallows of Greenwich Bay, Wickford Cove, and other shallow subembayments of the Bay where significant nutrient loads occur.

Comparisons of results between summer 2006 (wet early summer) and 2007 (very dry summer) indicate that the nuisance green algae such as Ulva lactuca may be more prevalent in wet summers when nutrients increase further due to nonpoint additions, while dryer conditions may be more conducive to other green and many red macroalgal species. Summer 2007 was noteworthy for the common and significant contribution of several red algal species on the western shore of the Bay below Conimicut Point, while the Providence River below the WWTFs still experienced significant nuisance algal growth. This was verified by both ground-truthed aerial photography (helicopter photo-surveys) and the amount of Ulva lactuca removed by the RIDEM from the Providence River shallows in 2006-2007. Nuisance Ulva blankets were especially thick in the mid and lower Providence River, although lesser amounts were also found in the shallows of Greenwich Bay, Wickford Cove, and other small shallow subembayments around the Bay. These areas outside the Providence River saw large amounts of red algae, including Gracilaria and Agardhiella as well as proliferation of other green seaweeds including Ulva linza.

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