Trophic Connectivity of Juvenile Snappers Inhabiting Back-Reef Nurseries in Belize

In tropical ecosystems, the productivity of a nursery is determined in part by the dynamics of organic matter delivered to or generated within that nursery area. Here, we address the issue of trophic connectivity for back-reef nurseries in Belize by identifying the source(s) of organic matter supporting three species of juvenile snapper: schoolmaster (Lutjanus apodus), gray snapper (L. griseus), and dog snapper (L. jocu). Stable isotope ratios (δ¹³C and δ¹⁵N) of producers and consumers were used to determine the relative contribution of local production vs. externally subsidized nutrient import for back-reef nurseries at inner, middle, and outer shelf locations (i.e., ecotones).  Producers and consumers were collected during both dry (February-May) and rainy (June-September) seasons at sites in the north (low freshwater inflow) and south (high freshwater inflow) to address spatial and temporal trends in nutrient delivery and trophic connectivity. Tissue δ¹³C values for benthic diatoms, seagrass, and mangroves showed no significant seasonal, regional, or ecotonal effects. However, δ¹³C values for POM collected at southern sites were significantly affected by both season and shelf position, with increasingly enriched values across the inshore-offshore gradient. This trend was distinct and consistent across both seasons at this site but was noticeably more pronounced during the rainy season. δ¹³C values for POM collected at northern sites showed a similar trend in enrichment across an inshore-offshore gradient, but here the trend was less pronounced and only present during the rainy season.  δ¹³C values for fish tissue were intermediate between the δ¹³C values for POM and benthic diatoms, but reflected the seasonal, regional, and ecotonal trends observed for POM, suggesting that trophic ecology and sources of organic matter for juvenile snappers in Belize are affected by both freshwater input and position of the nursery on the continental shelf.