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Satellite remote sensing of chlorophyll a in support of nutrient management in the Neuse and Tar-Pamlico River (North Carolina) estuaries
Keith, Darryl J. Satellite remote sensing of chlorophyll a in support of nutrient management in the Neuse and Tar-Pamlico River (North Carolina) estuaries. REMOTE SENSING OF ENVIRONMENT. Elsevier Science Ltd, New York, NY, 153:61-78, (2014).
The purpose of this report is to present information to the Division of Water Resources at the NC Department of Environmental Natural Resources (NCDENR) and the Albemarle-Pamlico National Estuary Partnership on the potential of using satellite information to identify time periods and locations along the Neuse River and Tar-Pamlico River estuaries where the North Carolina (NC) Water Quality standard for chlorophyll a (chl a) is exceeded. The standard is violated for concentrations greater than 40 μg/L and serves as the foundation of the nitrogen TMDL for the Neuse and Tar-Pamlico estuaries. Using satellite imagery (~300 m pixel resolution) provided by the European Space Agency (ESA) Medium Resolution Imaging Spectrometer (MERIS) instrument, the magnitude and extent of chl a concentrations were estimated at multiple temporal and spatial scales in these estuaries. Quantitative estimates (i.e., #of pixels > 40 μg/L as well as % of total pixels > 40 μg/L) and statistical analyses (mean, median, and 90th percentile values) of weekly, regional scale chlorophyll data are presented from 2006-2009 to determine periods of TMDL exceedances and NC water quality standards violations.
The North Carolina Environmental Management Commission (EMC) has adopted as a water quality standard that chlorophyll a concentration should not exceed 40 ug/L in sounds, estuaries and other slow-moving waters. Exceedances require regulators to develop a Total Maximum Daily Limit (TMDL) for nutrients in that water body. For the Neuse River, in 1996, load reductions were placed into North Carolina law to reduce the extent and duration of algal blooms (Session Law 1995, Section 572). In Phase II, the chlorophyll a criterion was used as the endpoint to manage total nitrogen concentrations, and TMDL compliance would be achieved if Chl a exceedances occurred in fewer than 10% of the samples collected in a specified area and time. For the Tar-Pamlico River, in 1989, Phase I consisted of the development of a nutrient trading framework, the development of an empirical model to estimate loading to the estuary and an evaluation of existing water treatment plants. Phase II consisted of refining model loading calculations and wetlands restoration. The application of remote sensing techniques to water quality assessment in the Neuse River estuary and Pamlico Sound began in the mid-1980s with the objective of using satellite or aircraft data to assist government agencies establish and monitor water quality (e.g., for TMDLs), enforcing water quality-related environmental regulations, and understanding the environmental impacts of the land-use practices in coastal waters. Remote sensing techniques have yet to be applied to the Tar-Pamlico estuary. It was agreed, during an October 2009 workshop, held to discuss incorporating satellite Chl a concentration into the NC decision process, that the high temporal and spatial resolution of the MERIS chl a product could be beneficial to the State of NC during any future development of water quality regulations based on “phytoplankton density”, to support retrospective analysis for the potential development of new nutrient regulations or revisions of the current TMDL, and to site future in-situ monitoring locations and detect unknown bloom events. In this study, the MERIS chlorophyll a data product was used to conduct a retrospective analysis of the NC standard for water quality and TMDL for total nitrogen in the Neuse and Tar-Pamlico River estuaries from 2006 – 2009. From field samples collected by the USEPA, NCDENR and the Lower Neuse Basin Association, a strong relationship was observed between in situ chlorophyll and total nitrogen in east coast estuaries (R2 = 0.62) and a moderate relationship (R2 = 0.49) in the Neuse River estuary. Using the high spatial resolution capability of MERIS, the total amount of estuarine acres impaired by chlorophyll over the four year period ranged from 269,319 acres in the Neuse River Estuary to 411, 620 acres in the Tar-Pamlico Estuary. From daily MERIS images, TMDL compliance and chlorophyll exceedances in the Neuse and Tar-Pamlico River estuaries were assessed at daily and annual time scales. The 90th percentile value of chlorophyll a concentrations was the statistic most useful for determining TMDL violations and to support environmental compliance monitoring.