Management Responses to Excess Nutrients

NBEP — Tue, 16 Dec 2008 00:59:43 +0000

The Rhode Island General Assembly passed legislation in 2004 requiring that nitrogen (a key nutrient controlling plant growth) be treated at all major RI sewage treatment plants by 2008 (RIGL § 46-12-2(f). The NBEP continues to work with state agencies like the RIDEM to reach this goal of limiting excess nitrogen entering the upper Bay. The latest projected nutrient decreases are available in a RIDEM report on the progress on nitrogen permit limits (RIDEM 2005 ).

Congress, through the Clean Water Act, requires that all waters be returned to fishable-swimmable conditions. Although negative water quality impacts such as hypoxia are the result of a complex interplay between physical parameters (stratification potential) and biological response to specific forms of nutrient loads (e.g., nitrate and ammonium), the only controllable aspect is the release of nutrients. Most researchers therefore argue that management responses should focus on N controls in estuarine systems (Paerl 2006, Piehler et al. 2004). The present negative impacts of excess nutrients, especially nitrogen, to the urban marine waters of the state are severe enough to fully support the actions of environmental agencies to control and decrease their release. The point sources like wastewater treatment plants are significant loads, and the treatment and removal at these centralized points is straightforward and very cost effective. Sewer line extension efforts such as in Warwick (Greenwich Bay) are also part of this type of management response. Where sewer extensions are not feasible, management of septic systems and phase out of cesspools will provide some help, although most septic systems do not remove nitrogen well. New advanced systems may be needed in sensitive zones where sewers are not possible. The most recent technologies for septic systems can remove up to 60% of the nitrogen (Costa et al. 2002).

Historical shellfish landings compared with previous era nitrogen loads indicate we can have a substantial resource population of filter-feeding shellfish at previous lower nutrient levels. The maximum yield of oysters peaked in the 1890s, strongly suggesting that the amount of nutrients and chlorophyll available was quite adequate for a significant filter-feeding population of important bivalves at a time when the Bay had a much lower nutrient loads than we will be able to reach with planned reductions. Oviatt (2008) has calculated that a 20 % decrease in nutrient load would not have detectable changes , while a 50 % reduction in nutrients would likely be detectable in terms of decreased chlorophyll and zooplankton based on regressions developed between these parameters and present nutrient levels. Any change in adult quahogs would not be detectable due to the high fishing pressure on this population. Oviatt (2008) recommends a survey of juvenile (sublegal sized) quahogs to track changes in this population. The expected maximal decrease in nitrogen based on WWTF denitrification is projected to be in the range of a 35% drop in Total Nitrogen load to the upper Bay (Pryor 2004). This would require the Worcester WWTF (UBWPAD) to denitrify under a permit limit that is presently being contested. It is unclear whether the projected level of actual nutrient decrease will have a clearly detectable change in these parameters. The maximum nutrient decrease from RI WWTFs is not expected until ~ 2012 when the Fields Point plant is ready to denitrify. It is expected that poor water quality conditions due to hypoxia (driven by excessive levels of chlorophyll) will lessen with a significant decrease in nutrient load (Oviatt 2008).

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Management Response to Excess Nutrients

Management Responses to Excess Nutrients