Projects

Characterizing stormwater nitrogen inputs to Mississippi’s coastal waters: A landscape approach

End Date: 02/01/12

Abstract

In the two year study proposed, rainwater and stormwater nitrogen along the Mississippi Sound will be characterized over a two year period by evaluating nutrient concentrations and the stable 15N composition of NH4 and NO3 in rainwater and stormwater. Selected stormwater sites specific to particularly levels of development have been chosen to span a land use / land cover gradient from the natural, undeveloped landscape to highly developed and hardened areas. More integrated stormwater samples representative of larger drainage areas characteristic of watershed level drainage will also be sampled from storm drain pipes/sloughs that deliver stormwater directly to Mississippi Sound to more accurately describe the actual stormwater that is delivered to the Sound.

We also propose to sample Mississippi Sound surface waters, oysters and macroalgae from three transects for N concentrations and 15N compositions to determine if rainwater and stormwater N is a major source of N for low trophic levels within the food web of Mississippi Sound. These 2 km transects will extend from nearshore to offshore in the Sound, beginning near the proposed stormwater sampling sites. These data will help to further evaluate the impact of stormwater on surface waters in Mississippi coastal waters and will contribute to sustainability studies and nutrient models currently being developed in the northern Gulf of Mexico. Such data would also be extremely useful in for future studies to identify potential stormwater “hotspots” in existing and rapidly developing coastal areas along the northern Gulf of Mexico.

We will test the hypothesis that stormwater nutrient concentrations and 15N values of dissolved inorganic nitrogen (DIN) will vary with the degree of development (i.e. hardened surface cover) due different physical and chemical processes that occur during stormwater transport through distinct landscape types. We will also test the hypothesis that stormwater N from hardened / highly developed areas will dominant the nitrogen pool in stormwater from larger drainage areas that contain mixed land use areas.  It is also hypothesized that stormwater from hardened drainage areas will have a larger zone of influence in Mississippi Sound receiving waters due to the more channeled and rapid delivery of N as well as the lack of a natural terrestrial buffer system (woodland and marsh) that is present at the natural undeveloped site. To test these hypotheses we will measure various forms of dissolved nitrogen concentrations and isotopic values in rainwater, stormwater, surface waters as well as in experimental macroalgae and oyster placed in the Mississippi Sound. The overall goal of the project is to determine the contribution of nitrogen from rainwater and stormwater to Mississippi coastal waters and to the living biota found in these waters.

Objectives

  1. To identify nutrient concentrations and 15N values of NH4 and NO3 in precipitation collected at two locations on the Mississippi Gulf coast throughout the 2 year study period.
  2. To identify the nutrient concentrations and 15N values of NH4 and NO3 in stormwater along a gradient of landscape development in coastal Mississippi throughout the 2 year period.
  3. To characterize changes in dissolved  nutrient concentrations and the 15N values of NO3 and NH4 in stormwater relative to rainfall from three distinct landscape types as well as from larger drainage areas that drain directly to the Sound and encompass more than one land-use type (a landscape characteristic common to many communities along the Northern Gulf Coast).
  4. To determine what fraction of stormwater delivered directly to the Sound originates from each general landscape type.
  5. To determine how stormwater N reacts in surface waters, i.e. is stormwater N advected into the Sound and transported some distance or is it rapidly diluted or removed from the water column due to biological uptake?
  6. To determine whether stormwater is be a major N source for primary producers (macroalgae) and primary consumers (oysters) in the Sound and if so, to determine how far the influence of stormwater nitrogen can be tracked into the Mississippi Sound from the mainland.

Methodology

Two “levels” of stormwater samples will be examined in this study. The first level will be very specific small sub-watershed landscape types which will encompass a gradient of landscape types common to southern Mississippi: completely hardened areas (highly developed), semi-hardened areas (residential and moderate commercial development), and unhardened undeveloped areas. The second level of stormwater samples will be collected directly from stormwater pipes that flow directly into Mississippi Sound.  Duplicate stormwater samples will be collected in wide mouth containers from each sampling site directly from stormwater ditches/pipes and placed on ice until return to the laboratory where they will be filtered through an precombusted GF/F filter then frozen until analysis for (NH4, NO3, NO2, DON, and PO4) and the 15N composition of NH4 and NO3 + NO2 is performed. Temperature and pH of each stormwater sample will also be measured at the time of collection to examine the expected loss of ammonium from stormwater due to volatilization.

Mississippi Sound surface waters and biota samples will be collected along three 2km long transects located near the stormwater sampling sites to determine if SW N can be traced into the local food web and if so, how far offshore the SW N can be traced. To this endeavor, we will place living oysters (Crassostrea virginica) at each site at the beginning of each sampling year (to begin in April) and periodically (approximately every 4 months) measure the 15N composition of the oysters. For each biota station (3 stations on 3 transect, 9 total), two trays of oysters will be placed at depth on the sediment in the Mississippi Sound and marked at the water surface with a GCRL buoy.  Six oysters will be sampled at the time of deployment and then approximately every three months at each station for its nitrogen content and15N composition. Oysters to be used for the study will be harvested from a commercially open oyster bed, placed on trays, covered with a mesh screen and then will then deployed at each site. Similar oyster trays are currently be used by other researchers at GCRL to evaluate oyster growth and distribution in the Sound so it is a proven field technique for oyster incubations.

To evaluate N assimilated by primary producers, red macroalgae (Gracilaria sp.) specimens will periodically (approximately every 4 months) be placed at each transect station a day before an anticipated rainfall event and will be allowed to “soak” at each site for 7 days after the rain event (as described by Costanzo et al., 2001). The entire specimen will then be harvested for 15N analysis. Living specimens will be incubated in situ in clear flow through chambers and suspended from the buoy line at each station. Red macroalgae for the study will be collected from the GBNERR (where it is abundant), and a sample for the analysis of the original 15N composition will be collected. Four such macroalgae incubations each year will be conducted at each station and will coincide with the oyster sampling described above. A similar technique using macroalgae that were placed at individual sampling sites and “soaked” in situ has been used by other researchers (Costanzo et al., 2001) with excellent results. 
Surface water samples and basic physical parameters (temperature, salinity, pH, and dissolved oxygen) will also be collected from each transect station prior to collecting the sample organisms and analyzed for nutrients (NH4, NO3, NO2, DON, and PO4) and the 15N composition of NH4 and NO3+ NO2. Water samples will be pneumatically pumped through precombusted glass fiber filters into Nalgene sample bottles, placed on ice until return to the lab where they will be frozen until analysis can be conducted.  Short transects were chosen for this project based on the PIs previous experience in a related study conducted in Sarasota Bay, where dissolved inorganic N from known stormwater sources were rapidly diluted below detection or taken up by biological activity within 1 km of the source (Dillon and Chanton, 2008). Short transects will provide the best resolution of stormwater mixing with receiving waters and is therefore the best approach for the chemical tracer work proposed here.
The biota, nutrient and DI15N samples will provide “snapshots” of different nitrogen pools in the Mississippi Sound at the times sampled and will allow temporal and spatial distribution of N from rainwater and stormwater sources to be evaluated. The relatively long term monitoring of oysters, the short term monitoring of macroalgae and the instantaneous monitoring of water column N pools will allow temporal variations in N sources to be evaluated at each station.

Rationale

Due to the rapid development that is currently underway along the northern Gulf of Mexico, it is essential to understand the impact of land use / land cover changes and subsequent changes in stormwater nutrient loads to coastal ecosystems which receive terrestrial runoff.  In the two year study proposed here, rainwater and stormwater nitrogen along the Mississippi Sound will be characterized over a two year period by measuring the nutrient concentrations and the stable 15N composition of NH4 and NO3 of rainwater and stormwater. Stormwater will be examined from small drainage scales specific to distinct landscape types to larger more generalized landscapes that encompass different land types within its boundaries. We also propose to determine if rainwater and stormwater N is a major sources of N for lower trophic levels within Mississippi Sound by measuring the 15N values of DIN in surface waters, macroalgae, and oysters along three 2 km transects that will extend into Mississippi Sound. These data will help to further evaluate the impact of stormwater on surface waters in coastal waters and will contribute to sustainability studies and nutrient models currently being developed in the northern Gulf of Mexico. Such data would also be extremely useful in for future studies to identify potential stormwater “hotspots” in existing and rapidly developing coastal areas along the northern Gulf of Mexico.

For More Information Contact: the MASGC Research Coordinator, Loretta Leist (Loretta.leist@usm.edu). 
Please reference the project number R/CCD-20.