Nitrogen Cycling, Primary Production, and Water Quality in the New River Estuary Defense Coastal/Estuarine Research Program (DCERP) Introduction: A key theme of the ongoing DCERP program is monitoring and research to understand the structure and function of the New River Estuary (NRE) and its surrounding watershed, and how they respond to external perturbations from Marine Corps Base Camp Lejeune (MCBCL) (population, development, training), regional (population, agriculture, swine operations), and natural (climate change, hurricanes, storms) stressors. The NRE is a shallow microtidal photic system with a long residence time; thus it is predicted to respond very differently to nutrient enrichment and other stressors than deeper, tidal estuaries. The system is surrounded by a watershed that includes MCBCL, the City of Jacksonville, and rural agriculture including confined animal feeding operations. The main focus of DCERP in the NRE is on nitrogen cycling and primary production, as these combine to determine system water quality and attainment/violation of state water quality standards. Research within these areas is being conducted across multiple DCERP modules but is being integrated into Watershed and Estuarine Simulation Models (WSM, ESM Fig. 1) to predict system response to the stressors listed above and guide management to maintain system health and function. Figure. 1. Schematic of the DCERP Estuarine Simulation Model (ESM) showing major state variables, processes, and forcing functions which are being quantified by DCERP research. Key terms are defined in the text. Temperature (TEMP) affects most processes in the model and is not connected to reduce diagram complexity.
Nitrogen Cycling: The fate and transport of nitrogen (N) in coastal regions is a key determinant of the function of these ecosystems, as it is most often the supply of N that limits the production of organic matter that serves as the base of the estuarine food web and determines rates of material (carbon, nutrients and oxygen) cycling and resultant water quality. Understanding the natural and anthropogenic impacts on the delivery of nitrogen to coastal waters is essential for effective management of water quality, habitat and fisheries. Nitrogen enters the NRE from the watershed in streams and groundwater, from atmospheric deposition (both wetfall and dryfall), from N 2 fixation by sediment microbes, and in incoming water from Onslow Bay due to physical exchanges of water (Fig. 1). Once in the estuary, N (as well as phosphorus, P, and other nutrients) are subject to uptake by phytoplankton (PHYTO) and benthic microalgae (BMA) to support primary production, hydrodynamic circulation and flushing, and denitrification (N only). N that has been incorporated into biomass is subsequently released due to respiration of organic matter in the water column (R WC ) and sediments. In the DCERP project each major N source, transformation, and impact in Figure 1 is being addressed as part of the Aquatic/Estuarine, Coastal Wetlands and Atmospheric modules as follows, and is being synthesized into the ESM for a synthetic, system-level examination of N cycling: Atmospheric module: Atmospheric deposition as a source of N to the NRE and its watershed (Air-2) Coastal Wetlands module: Assessing NRE wetlands as sources or sinks of N (CW-3) Assessing impacts of N on coastal wetland productivity and structure (CW-1) Aquatic/Estuarine module: Riverine loading of N, and associated freshwater and phosphorus (P) (monitoring) Headwater stream loading of N and P (including base flow measurements that capture shallow groundwater inputs) (monitoring, AE-2) Photic shoal N cycling including remineralization, denitrification, N fixation, benthic N fluxes, and the link between N and production by benthic microalgae (AE-3) Estuarine water column N cycling including phytoplankton nitrogen utilization, links to phytoplankton community structure and eventual impacts of nitrogen loading on oxygen dynamics (monitoring, AE-1). Primary Production: Primary production is the process of organic carbon formation that sustains food webs and fuels microbially-mediated carbon, nutrient (N, P, trace metals) and oxygen (O 2 ) cycling in the aquatic environment. The vast amount of primary production is conducted by photosynthetic plants. In the NRE, a bulk of the primary production is mediated by planktonic and benthic microalgae, while in the fringing marsh, wetland terrestrial and barrier systems, higher plants (macrophytes, woody plants) dominate this process. Nutrient (N and P) inputs, light availability (turbidity) and advective processes, including flow, tides and flushing, interactively control the rate of primary production and the community composition of primary producers. While nutrient inputs are essential for supporting primary producers, excessive
nutrient supplies can lead to excessive production (eutrophication), which can adversely affect water quality and habitat condition. Examples include harmful (toxic, food web-altering) algal blooms, hypoxia and anoxia resulting from decaying algal material, decreased light availability (essential to support benthic primary production and nutrient uptake), loss of habitat for finfish and shellfishing, recreational and training activities. While nutrient supplies from external (i.e. atmospheric, surface and subsurface terrestrial and oceanic), and internal (within system recycling) sources play key regulatory roles, physical factors such as freshwater discharge, flushing (water residence time) and transparency modulate primary production by determining the location, amounts and fates of primary production and the composition of microalgal and higher plant species conducting this process. Furthermore, top down grazing by microbes, invertebrates and higher animals (fish) exerts another pressure on primary producer biomass and composition. In the microtidal New River Estuary, freshwater discharge and resulting flushing rates (water residence time) play fundamentally-important roles in controlling the amount and composition of planktonic primary producers. This is of critical importance from ecological function and ecosystem health perspectives. Freshwater discharge also controls nutrient inputs and rates of nutrient cycling. Clearly, there is a strong interaction between nutrient delivery and flushing characteristics of the estuary in terms of the types (including harmful species) and amounts (including exceedances the NC State allowable planktonic primary producer biomass, which is 40 µg L -1 chlorophyll a, the primary indicator of microalgal biomass). As a rule, poorly flushed estuarine waters with high nutrient concentrations tend to have high incidences of algal blooms and exceedances, while fast flowing, highly-flushed waters (even those high in nutrients) will exhibit relatively low amounts of biomass per amount of nutrients supplied. Whereas the primary response to nutrient enrichment in deep estuaries are phytoplankton blooms and resulting eutrophication, we expect that the NRE will be more resilient. Benthic production, accompanied by denitrification, is likely to modulate the effects of nutrient enrichment; however, these processes are regulated by residence time, light and salinity, which vary during episodic storm events, and light availability, affected by erosion and resuspension in response to both natural and anthropogenic perturbations such as MCBCL training activities and development. It is therefore important that both chemical (i.e. nutrient loads and recycling rates) and physicalhydrological (including freshwater discharge, flow, residence time and turbidity/transparency) factors be integrated in our models aimed at predicting rates of productivity, biomass and compositional responses of primary producers in the river, estuary and adjacent coastal waters. In the DCERP project each of the key processes centered around primary production and its controlling factors in Figure 1 are being assessed in the Aquatic/Estuarine and Coastal Wetlands modules as follows or as stated above for nitrogen. These processes are being synthesized into the ESM for a synthetic, system-level examination of primary production, water quality, and response of the system to nutrient loading and other stressors. Aquatic/Estuarine module: Total biomass, community composition, productivity, and nutrient limitation by phytoplankton (Monitoring, AE-1)
Total biomass, community composition, productivity, and effects of light and nutrients on benthic microalgae (AE-3) Total rates of water column, benthic, and system metabolism (AE-3) Flushing of NRE phytoplankton (and nutrients) using a simplified box model (AE-3) and high resolution ADCIRC model Coastal Wetlands module: NRE marsh biomass, productivity, and rates of accretion/erosion (monitoring, CW-1) Response of NRE marshes to N and P additions (CW-1) Synthesis: The WSM and ESM are synthesizing several aspects of DCERP monitoring and research across multiple modules (Fig. 2). In addition to those discussed above, the modeling is synthesizing meteorological data from the Atmospheric module (e.g. wind, precipitation, photosynthetically active radiation PAR), GIS and landuse data, and the wide variety of estuarine water quality data being collected by the AE module. The goal of this synthetic work is to tie these projects together and scale up to the entire estuary to provide an overall understanding of NRE structure, function, and response to stressors. Once calibrated to measured conditions, a series of simulations will be performed to assess watershed and NRE response to a variety of MCBCL, regional, and natural stressors, with a focus on predicted watershed nutrient delivery, estuarine primary production, and water quality. These models will form the basis of decision-support tools for use by the base and regional managers for setting development priorities and nutrient targets (e.g. Total Maximum Daily Loads, TMDLs) for maintaining compliance with North Carolina s 40 µg L -1 standard for chlorophyll-a. What is the value of these data and models? These models will form the basis of decisionsupport tools for use by MCBCL and regional managers for setting development priorities and nutrient targets (e.g. Total Maximum Daily Loads, TMDLs) for maintaining compliance with North Carolina s 40 µg L -1 standard for chlorophyll-a. All coastal land managers must consider their decisions in the context of the coastal ecosystem. MCBCL has been a tremendous land steward, but its continued growth will have to account for possible impacts on coastal resources. Our data and models are directly applicable to making informed, sound decisions about growth of the base. The following examples illustrate the utility of DCERP data and modeling tools to the base: MCBCL is currently planning to build a new barracks area. DCERP data can be used to assess current water quality conditions downstream of this development (AE-1, AE-3) and the likely impact on nutrient and sediment loads (AE-2). The WSM-ESM can simulate this development scenario and predict resulting changes in loads, estuarine water quality, and violations of the state chlorophyll standard. Targeted development scenarios can be run anywhere in the watershed and used to determine the appropriate location, scale, and density of development. With future development, MCBCL may need to increase capacity at its sewage treatment plant. DCERP data can be used to place the increased nutrient loads in context within the overall NRE nutrient budget, and to assess the potential for increased inputs to impact the system. The ESM can be run to forecast the impact of
this expansion on water quality and to quantify the potential for violation of the state chlorophyll standard. Given that historical chlorophyll data indicate violations of the state standard in the NRE, the state is likely to impose a TMDL which would require regional and base managers to set and meet nutrient load reduction targets. The WSM-ESM can be used to determine the required magnitude of these reductions, and to partition them between regional and base sources. Figure. 2. Summary of within and cross-module linkages of data with the watershed and estuarine simulation models.