Coastal oceans, Great Lakes, and estuaries are complex and highly dynamic ecosystems shaped by coupled interactions among the physical, chemical, and biological processes associated with their terrestrial, freshwater, oceanic, and benthic components. These ecosystems are also highly productive, and therefore of great importance to society. Over time scales relevant to managers (i.e., the human life span) there are two overarching forces that are capable of perturbing the form, function, and ecosystem services provided by these systems: non-climatic human activities and climate variability and change. The combined effects of non-climatic and climatic stressors, interannual climate variability, and climate change can dramatically alter these environments causing eutrophication, modifying habitats, and changing basic environmental conditions, such as salinity, temperature, and alkalinity. Therefore, sensitivity of biophysical water conditions in the Reserve System represents a measure of dynamic responsiveness to climate change.
The connectedness between empirical water quality variables, such as temperature and specific conductivity, and the climate variables of precipitation and air temperature is examined in this report. An assessment of the responsiveness of springtime NERRS site-specific water quality variables to springtime atmospheric temperature and rainfall fluctuations across the NERRS was conducted. The main objective of this analysis is to evaluate the relative sensitivity of select water quality variables at each reserve to changes in climate variables. The secondary objective is to assess the most sensitive climate to water quality variable relationships across the NERRS, and to look for trends in sensitivity relative to reserve or water quality station characteristics, particularly land use and water drainage characteristics. Spring was chosen because, generally speaking, the annual growing season begins in spring due to increasing sunlight levels and warming temperatures. As such, variability in annual springtime rainfall and temperatures can result in dramatic changes in the biological structure and function of estuaries that influence biophysical processes and conditions throughout the rest of the year.
KEY FINDINGS
An NERRS Biophysical Sensitivity Index (BpSI) was developed to assess the relationship of climate and water quality at the reserve level. The result was an index that compared the reserves on a relative scale, ranging from Very High to Very Low biophysical sensitivity. The BpSI summarizes the annual spring atmospheric temperature and rainfall data at each reserve regressed against each water quality variable (water temperature, turbidity, water conductivity, pH, and dissolved oxygen) in order to characterize the strength of each reserve’s climate-biophysical relationship.
- Reserves that were characterized as having Very High and High biophysical sensitivity relative to all the reserves in the NERRS were scattered around the country. Biophysically sensitive reserves included Tijuana River NERR (CA), Sapelo Island NERR (GA), ACE Basin NERR (SC), and Waquoit Bay NERR (MA).
- The Tijuana River Reserve (CA) had the highest average reserve BpSI (4.8) and its high biophysical sensitivity corresponds with its heavily developed, urbanized watershed where the water has been channelized.
- Sapelo Island (GA) and ACE Basin (SC) reserves have BpSI values in the Very High and High categories, respectively, and co-occur in the Southeast. Both reserves contain extensive salt marshes and are relatively undeveloped. Seven of the eight water quality monitoring stations in these two reserves are located in tidal creeks and their high biophysical sensitivity scores imply that the salt marsh tidal creek habitats in these two reserves are sensitive to climate-induced increases in temperature and precipitation.
- Waquoit Bay Reserve (MA) has a High BpSI value relative to all the other reserves due primarily to a strong climate-related linkage between air and water temperature. In contrast, Waquoit Bay Reserve’s nearest neighbor, Narragansett Bay NERR (RI), had the second lowest BpSI value. This difference in biophysical sensitivity scores between these geographically close reserves indicates that local conditions are very important in determining climate-related biophysical sensitivity at these two reserves and possibly across the Reserve System.
- Eight of the nine reserves falling within the Moderate biophysical sensitivity group were located on the East Coast. Otherwise BpSI values did not show any distinct regional trends.
- The BpSI developed and used in this study separated reserves in an informative, relative scale, providing a useful foundation for exploring more detailed climate-biophysical relationships in the NERRS. The strongest individual relationships were found for the comparison of air temperature to water temperature and the comparison of precipitation to specific conductivity. The connection of climate variation to water temperature and saltiness is important, as these two water quality variables may have direct influences on estuarine natural resource populations.
- An important component of the variability seen in reserve sensitivity may be the result of the varying placement of the water quality monitoring stations in each reserve, which were originally established to assess targeted environmental gradients. It is recommended that the NERRS program evaluate different strategies for more targeted climate change monitoring at the reserve level.