NITROGEN ISOTOPE RECORDS IN FRESHWATER MUSSELS REFLECT WATERSHED LAND USE PATTERNS AND STREAM DISCHARGE

Measurements of nitrogen isotope ratios (δ¹⁵N) in the organic fraction of mollusk shells are increasingly applied to assess historical changes to anthropogenic nutrient input in estuaries and to track natural fluctuations of nitrogen in coastal waters. Recent proxy validation research supports similar applications in freshwater Unionid mussel shells, but fewer such analyses have been made despite the fact that anthropogenic impacts to their ecosystems are extensive and often began prior to instrumental monitoring. Analysis of sub-fossil and archaeological shells may reveal historical pollution data useful to guide remediation and restoration. Here we report δ¹⁵N data measured in Unionid mussel shells from the southern clubshell (Pleurobema decisum), Alabama orb (Cyclonaias asperata), and yellow sandshell (Lampsilis teres). These mussels grew in three watersheds across Alabama, USA, with differing land use patterns, including forest, farmland, and urban. Prior published research demonstrated that soft tissues and periostracum from mussels that grew in agricultural watersheds contain heavier δ¹⁵N values, likely reflecting effects of increased animal waste inputs and fertilizer runoff. We find that shell δ¹⁵N values follow this same pattern, thus supporting the analysis of ancient shells as potential sources of historical baseline data to assess later pollution. Likewise, δ¹⁵N values in shells from urban watersheds display generally heavier values than even the shells from the farmland watershed. These values varied in tandem with discharge measured from nearby stream gages. Thus, sclerochronological sampling may reveal sub-annual patterns of nitrogen variation to watersheds and may detect impacts of urbanization on runoff patterns. These results support the use of mussel shell δ¹⁵N values to track anthropogenic impacts to streams. Unionid mussels can live more than 100 years, are archived in museums, and may be well-preserved in archaeological sites of suitable sediment chemistry, thus long-term time series records can be constructed.