Establishing an Anthropogenic Nitrogen Baseline Using Native American Shell Middens

Narragansett Bay, Rhode Island, has been heavily influenced by anthropogenic nutrients for more than 200 years. Recent efforts to improve water quality have cut sewage nitrogen (N) loads to this point source estuary by more than half. Given that the bay has been heavily fertilized for longer than monitoring programs have been in place, we sought additional insight into how N dynamics in the system have historically changed. To do this, we measured the N stable isotope (δ15N) values in clam shells from as early as 3000 BP to the present. Samples from Native American middens were compared with those collected locally from museums, an archeological company, and graduate student thesis projects, during a range of time periods. Overall, δ15N values in clam shells from Narragansett Bay have increased significantly over time, reflecting known patterns of anthropogenic nutrient enrichment. Pre-colonization midden shell δ15N values were significantly lower than those post-European contact. While there were no statistical differences among shells dated from the late fifteenth century to 2005, there was a significant difference between 2005 and 2015 shells, which we attribute to the higher δ15N values in the effluent associated with recent sewage treatment upgrades. In contrast, the δ15N values of shells from the southern Rhode Island coast remained constant through time; while influenced by human activities, these areas are not directly influenced by point-source sewage discharge. Overall, our results show that this isotope technique for measuring δ15N values in clam shells provides useful insight into how N dynamics in coastal ecosystems have changed during thousands of years, providing managers vital historical information when setting goals for N reduction

Use of historical isoscapes to develop an estuarine nutrient baseline

Coastal eutrophication is a prevalent threat to the healthy functioning of ecosystems globally. While degraded water quality can be detected by monitoring oxygen, nutrient concentrations, and algal abundance, establishing regulatory guidelines is complicated by a lack of baseline data (e.g., pre-Anthropocene). We use historical carbon and nitrogen isoscapes over ~300 years from sediment cores to reconstruct spatial and temporal changes in nutrient dynamics for a central California estuary, Elkhorn Slough, where development and agriculture dramatically enhanced nutrient inputs over the past century. We found strong contrasts between current sediment stable isotopes and those from the recent past, demonstrating shifts exceeding those in previously studied eutrophic estuaries and substantial increases in nutrient inputs. Comparisons of contemporary with historical isoscapes also revealed that nitrogen sources shifted from a historical marine-terrestrial gradient with higher δ15N near the inlet to amplified denitrification at the head and mouth of the modern estuary driven by increased N inputs. Geospatial analysis of historical data suggests that an increase in fertilizer application – rather than population growth or increases in the extent of cultivated land – is chiefly responsible for increasing nutrient loads during the 20th century. This study demonstrates the ability of isotopic and stoichiometric maps to provide important perspectives on long-term shifts and spatial patterns of nutrients that can be used to improve management of nutrient pollution

Patterns in stable isotope values of nitrogen and carbon in particulate matter from the Northwest Atlantic Continental Shelf, from the Gulf of Maine to Cape Hatteras

Stable isotope measurements of nitrogen and carbon (δ15N, δ13C) are often used to characterize estuarine, nearshore, and open ocean ecosystems. Reliable information about the spatial distribution of base-level stable isotope values, often represented by primary producers, is critical to interpreting values in these ecosystems. While base-level isotope data are generally readily available for estuaries, nearshore coastal waters, and the open ocean, the continental shelf is less studied. To address this, and as a first step towards developing a surrogate for base-level isotopic signature in this region, we collected surface and deep water samples from the United States’ eastern continental shelf in the Western Atlantic Ocean, from the Gulf of Maine to Cape Hatteras, periodically between 2000 and 2013. During the study, particulate matter δ15N values ranged from 0.8 to 17.4 ‰, and δ13C values from -26.4 to -15.6 ‰ over the region. We used spatial autocorrelation analysis and random forest modeling to examine the spatial trends and potential environmental drivers of the stable isotope values. We observed general trends towards lower values for both nitrogen and carbon isotopes at the seaward edge of the shelf. Conversely, higher δ15N and δ13C values were observed on the landward edge of the shelf, in particular in the southern portion of the sampling area. Across all sites, the magnitude of the difference between the δ15N of subsurface and surface particulate matter (PM) significantly increased with water depth (r2 = 0.41, df = 35, p < 0.001), while δ13C values did not change. There were significant positive correlation between δ15N and δ13C values for surface PM in each of the three marine ecoregions that make up the study area. Stable isotope dynamics on the shelf can inform both nearshore and open ocean research efforts, reflecting regional productivity patterns and, even possibly, large-scale climate fluctuations

A marked gradient in δ\u3csup\u3e13\u3c/sup\u3eC values of clams Mercenaria mercenaria across a marine embayment may reflect variations in ecosystem metabolism

Although stable isotopes of organic carbon (δ13C) are typically used as indicators of terrestrial, intertidal, andoffshore organic carbon sources to coastal ecosystems, there is evidence that δ13C values are also sensitive to in situecosystem metabolism. To investigate this phenomenon, we examined δ13C values of filter-feeding hard clams Mercenaria mercenaria from 13 locations in Greenwich Bay, a sub-estuary of Narragansett Bay, Rhode Island (USA). The δ13C values ofthe clams showed a marked linear gradient of 2‰ over the 4 kmlength of Greenwich Bay (-19 to -17‰), from lower δ13C values in the inner bay to higher values at the mouth, where Greenwich Bay joins Narragansett Bay proper (R2 = 0.94, p \u3c 0.0001). This is in contrast to previous work that has shown that δ13C values of clams in Narragansett Bay proper (over 40 km long) are homogenous (mean × SD, -16.8 × 0.6 ‰, n = 247). Mean daily pH, temperature, and salinitydata from 2 fixed monitoring stations were used to estimate aqueous CO2(CO2(aq)) concentrations in the surroundingwater. CO2(aq) concentrations were higher in inner Greenwich Bay than immediately outside of the bay, suggesting that the dissolved inorganic carbon sources supporting phytoplankton production are quite different across the bay. The outer Greenwich Bay clams appear to feed on Narragansett Bay phytoplankton with higher δ13C values that are grown in a higher pH, more bicarbonate-rich environment. In contrast, the inner Greenwich Bay clams may feedon phytoplankton grown in lower pH water with a greater availabilityof CO2(aq). The lower δ13C of CO2(aq) relative to HCO3 - is reflected in the phytoplankton and in the clams that feed on them. Our work suggests that δ13C values may be sensitive to changes in inorganic C in estuarine systems, which may confound attempts to use stable isotopesto identify organic carbon sources. © Inter-Research 2010

Nitrogen control through decentralized wastewater treatment: Process performance and alternative management strategies

Decentralized or onsite wastewater treatment (OWT) systems have long been implicated in being a major source of N inputs to surface and ground waters and numerous regulatory bodies have promulgated strict total N (TN) effluent standards in N-sensitive areas. These standards, however, most of which have effluent limitations of \u3c10. mg/L TN, were generally developed without data on treatment performance and attainable compliance levels of operating OWTs designed to remove N. This paper reviews OWT technologies that rely on preanoxic or postanoxic denitrification, or simultaneous nitrification-denitrification, and frequently include compact, mechanized components. TN effluent data from 20 OWTs in 3 long-term N removal demonstration projects in Florida, Oregon, and New Zealand are analyzed and compared with the performance of 15 centralized N removal treatment plants from the US and Canada. A reliability and stability analysis shows that only one of the 20 OWTs approaches the reliability and stability of centralized plants, and can comply with a \u3c10. mg/L TN effluent standard with a 99% probability; all of the remaining 19 OWTs have a \u3c50% probability of compliance. The lower reliability of OWTs, many of which are energy-intensive, scaled-down models of centralized plants, is due to the inherent variability of decentralized wastewater characteristics and the challenges of operationally controlling N removal processes at the level of residences. However, the small footprint (required land area) of these compact designs offers important opportunities for retrofitting OWTs on small lots, in shoreline developments where land is at a premium and where communities wish to foster and sustain compact, village developments that reflect smart growth strategies. Other approaches to decentralized N management emphasizing passive, robust, ecologically engineered designs are reviewed and include natural wastewater treatment systems such as single pass sand filters with denitrifying bioreactors, which performed better than any other OWT technology; shallow trenches and drip irrigation for denitrification or plant N uptake in the carbon-rich root zone; denitrification beds/layers installed down gradient from effluent plumes; and the consideration of watershed N sinks in estimating the risks of N loading to receiving waters. These alternative approaches require further research and development, but can offer alternatives or additional treatment to mechanized OWTs. More comparative studies of long-term operation of OWTs under field conditions in other parts of the world are needed to further quantify performance capabilities. © 2010 Elsevier B.V

Testing sample stability using four storage methods and the macroalgae Ulva and Gracilaria

Concern over the relative importance of different sample preparation and storage techniques frequently used in stable isotope analysis of particulate nitrogen (δ15N) and carbon (δ13C) prompted an experiment to determine how important such factors were to measured values in marine organisms. We stored the marine macroalgae Ulva and Gracilaria in four different ways and analyzed replicates every three months over the course of a year to assess treatment effects on stability. Treatments consisted of algae dried at 65°C, ground to a powder, and stored in a desiccator until analysis; algae left in a drying oven or in a freezer and processed (dried and ground) just prior to analysis, as well as some dried, ground samples kept out in the lab and reanalyzed quarterly for 12 months. Concurrently, to assess the ecological range in isotope values over the course of a year, samples were freshly collected from the same location and analyzed along with the other treatments at each time step. Neither storage technique nor time had an impact on either δ15N or δ13C values or the %N and %C of the algae tissues. There were clear and consistent differences between species and some large seasonal differences in the freshly collected samples. The interspecies differences and seasonal ranges of values underscore the stability associated with method and duration of sample storage

Anthropogenic enhancement of Egypt\u27s Mediterranean fishery

The highly productive coastal Mediterranean fishery off the Nile River delta collapsed after the completion of the Aswan High Dam in 1965. But the fishery has been recovering dramatically since the mid-1980s, coincident with large increases in fertilizer application and sewage discharge in Egypt. We use stable isotopes of nitrogen (δ15N) to demonstrate that 60%–100% of the current fishery production may be from primary production stimulated by nutrients from fertilizer and sewage runoff. Although the establishment of the dam put Egypt in an ideal position to observe the impact of rapid increases in nutrient loading on coastal productivity in an extremely oligotrophic sea, the Egyptian situation is not unique. Such anthropogenically enhanced fisheries also may occur along the northern rim of the Mediterranean and offshore of some rapidly developing tropical countries, where nutrient concentrations in the coastal waters were previously very low

Carbon Stable Isotope Values in Plankton and Mussels Reflect Changes in Carbonate Chemistry Associated with Nutrient Enhanced Net Production

Coastal ecosystems are inherently complex and potentially adaptive as they respond to changes in nutrient loads and climate. We documented the role that carbon stable isotope (δ13C) measurements could play in understanding that adaptation with a series of three Ecostat (i.e., continuous culture) experiments. We quantified linkages among δ13C, nutrients, carbonate chemistry, primary, and secondary production in temperate estuarine waters. Experimental culture vessels (9.1 L) containing 33% whole and 67% filtered (0.2 μm) seawater were amended with dissolved inorganic nitrogen (N) and phosphorous (P) in low (3 vessels; 5 μM N, 0.3 μM P), moderate (3 vessels; 25 μM N, 1.6 μM P), and high amounts (3 vessels; 50 μM N, 3.1 μM P). The parameters necessary to calculate carbonate chemistry, chlorophyll-a concentrations, and particulate δ13C values were measured throughout the 14 day experiments. Outflow lines from the experimental vessels fed 250 ml containers seeded with juvenile blue mussels (Mytilus edulis). Mussel subsamples were harvested on days 0, 7, and 14 and their tissues were analyzed for δ13C values. We consistently observed that particulate δ13C values were positively correlated with chlorophyll-a, carbonate chemistry, and to changes in the ratio of bicarbonate to dissolved carbon dioxide (HCO3-:CO2). While the relative proportion of HCO3- to CO2 increased over the 14 days, concentrations of each declined, reflecting the drawdown of carbon associated with enhanced production. Plankton δ13C values, like chlorophyll-a concentrations, increased over the course of each experiment, with the greatest increases in the moderate and high treatments. Trends in δ13C over time were also observed in the mussel tissues. Despite ecological variability and different plankton abundances the experiments consistently demonstrated how δ13C values in primary producers and consumers reflected nutrient availability, via its impact on carbonate chemistry. We applied a series of mixed-effects models to observational data from Narragansett Bay and the model that included in situ δ13C and percent organic matter was the best predictor of [HCO3-]. In temperate, plankton-dominated estuaries, δ13C values in plankton and filter feeders reflect net productivity and are a valuable tool to understand the production conditions under which the base of the food chain was formed