SPATIAL AND TEMPORAL PATTERNS IN NUTRIENT STANDING STOCK AND MASS-BALANCE IN RESPONSE TO LOAD REDUCTIONS IN A TEMPERATE ESTUARY

The addition of excess organic matter into a system, commonly referred to as eutrophication (Nixon, 1995), is a widespread problem in estuaries throughout much of the world. To combat this trend, many management agencies are imposing regulations limiting the amount of nutrients (nitrogen and phosphorus) which can be discharged into coastal waters through wastewater treatment and agriculture. In 2005, the Rhode Island Department of Environmental Management (RIDEM) enacted legislation mandating that wastewater treatment facilities (WWTF) discharging their effluent into Narragansett Bay and its tributaries reduce the concentration of nitrogen in their effluent. This legislation will reduce wastewater nitrogen loading to the bay by 50% by 2014 with the ultimate goal of improving water quality, reducing hypoxia, and restoring lost ecosystem services (e.g. seagrass) to the bay. Early stages of this reduction took place between 2005-2009, reducing loadings at 11 WWTF’s which discharge into the bay from 16-20mg/l total nitrogen to either 8 or 5mg/l. Response of other estuaries to similar reductions in loading has been varied and complex, with relatively few ecosystems showing straightforward linear reductions in concentration, productivity, and chlorophyll with reduced load. The overall goal of this study is to quantify the impact of these initial loading reductions on the standing stock (Chapter 1), seasonal cycling (Chapter 2), and mass-balance (Chapter 3) of nitrogen and phosphorus in Narragansett Bay. To accomplish this goal, we first reviewed data from a five-year study of surface nutrient concentration at 13 stations throughout Narragansett Bay (Chapter 1). Because Narragansett Bay is aligned along a north-south gradient of decreasing urbanization and most sources of nutrients to the bay are located in or around the city of Providence, at the head of the estuary, we can establish down-bay relationships of nutrient constituents to see how their concentrations change spatially throughout the bay, and compare these relationships to past studies. We can also use established volume relationships to estimate the total standing stock of nutrients in the bay at any given time, and compare how this changes over the course of a year during the present survey and during past surveys. In response to a 30% reduction in the total annual load of dissolved inorganic nitrogen from all sources, which corresponds to a 17% reduction in total nitrogen, we saw measurable reductions in downbay concentrations and standing stocks approximately on par with these reductions. Phosphorus concentrations in the bay have declined dramatically (30-50%) in part due to recent loading reductions, but also in part due to management action in the 1980’s and 1990’s to remove phosphates from detergents and industrial surfactants. We also see changes in the way nitrate, nitrite, and ammonium are used on a downbay gradient, which we hypothesize are related to the loading reductions. In order to fully understand the impact of load reductions on the ecosystem, we must also consider how the nutrients in the system have changed over the long-term, both in terms of annual cycling, and in terms of response to changing climate in the bay. This analysis constitutes the second chapter of the dissertation. Over the last 50-100 years, Narragansett Bay has grown measurably warmer, and weather patterns have changed, bringing increased cloud cover, more storms, and more precipitation. All of these changes impact the way nutrients enter the bay, and the way phytoplankton use the nutrients. We examined the impact of these potential changes using a long-term weekly dataset of nutrient concentrations collected by the MERL lab at the University of Rhode Island Graduate School of Oceanography since 1978. We use both conventional statistics and a state-space model formulated in the computing language R (SSPIR). Our results show virtually no long-term trend or change in timing of seasonal cycling of nutrients or chlorophyll. However, we do see changes in the seasonal patterns of concentration of both nutrients and chlorophyll at the GSO station, with measurable changes in cumulative distribution function for phosphate, silicate, ammonium, and chlorophyll. We also observe statistically significant reductions over the course of the time series for nitrate, nitrite, ammonium, and phosphate, though it is difficult to ascribe causality to these changes. Model results were largely inconclusive, but show a marginally significant intervention effect attributable to the loading reduction in the ammonium signal at the GSO dock, with no significant long-term trend observed for any analyte. Finally, we conduct a mass-balance nutrient budget assessment for nitrogen and phosphorus in Narragansett Bay (Chapter 3). Mass-balance is a common way of tracing the sources, sinks, and reservoirs of nutrients in a system, and seeing how these components might change with time. Nutrient budgets for Narragansett Bay have been compiled approximately every decade, but recent and future loadings compel a reanalysis to determine how the system is responding to initial stage reductions. We see a reduction in WWTF loading to the bay of just over 100 million moles of nitrogen and 4 million moles of phosphorus, which constitutes about 20 and 16 percent of the net annual load of nitrogen and phosphorus from all sources. However, much of this reduction is realized in tributary rivers, and variable riverine abatement rates in those rivers mean that some of the net reduction is not felt by the bay proper. Furthermore, evidence from literature suggests that changes in bay sediment net denitrification rate may be offsetting some or all of the loading reductions