Development of a stable-isotope constraint system for estuarine food-web models

Since the mid-1970s, stable carbon isotopes have been used extensively to address diverse questions related to estuarine carbon cycling. Initially, the isotopic approach was used to identify sources of organic matter and to determine their fate in estuarine food webs. Results obtained with this technique were qualitative; however, more recent attempts to quantify sources or processes have typically relied on simple 2 or at most 3 end-member mixing models. The key assumption is that sources can be defined by a discrete value. This assumption often does not hold and, more impor- tantly, the isotopic variation observed within sources contains significant information. In contrast, inverse-modeling of estuarine carbon cycling has the advantage that ranges of field and laboratory measurements and historical information can be utilized by this method. We used the inverse approach to revisit the issue of a headwater delta's contributions to an estuarine food-web under con- ditions of low freshwater inflow and after a flood-induced inundation of the delta (Rincon Delta, dis- charging into Nueces Bay, Texas, USA). We illustrate how isotopic constraints enhance results obtained with more traditional constraint systems. The results suggest that although benthic infauna production is stimulated by increased water flow and delta freshwater inundations, most of the allochthonous carbon is exported from Nueces Bay.