UNCOVERING THE PAST: UTILIZING INVERTEBRATE SUB-FOSSIL ASSEMBLAGES FROM BELIZEAN LAGOONAL REEFS TO DETERMINE DRIVERS AND TIMING FOR CARIBBEAN REEF ECOSYSTEM DECLINE

Caribbean coral reef ecosystems have rapidly declined in recent decades due to climate change and local human stressors. Although we have robust data sets following the onset of large-scale monitoring in the late 1970s, less is known about reef health before the 1970s. Furthermore, the majority of this monitoring work has focused on organisms like corals, parrotfish, and echinoderms. Much less is known about how changing environments are reflected in other important coral reef community members (e.g. molluscs). Reef matrix cores can help fill this knowledge gap, as they provide records of reef communities and environments over the past centuries to millennia, prior to large-scale human disturbance. To obtain a more accurate baseline of reef ecosystem structure and functioning and to track ecosystem change over the past one and a half millennia, we assessed changes in the taxonomic and functional group composition of sub-fossil assemblages of bivalves, gastropods, and urchins preserved in 3.5m-long cores from three reefs within the central lagoon of Belize. Bivalve and gastropod composition was assessed via relative abundance of shells, while urchin composition was assessed via the accumulation rate of spines measured by weight. These cores record a shift in the dominant substrate relationship in bivalves from epifaunal to infaunal over time, indicating a loss of hard substrate (i.e. corals). In contrast, urchin composition shows little change through time, with the currently-common Echinometra spp. dominating throughout the cores. Microalgivore gastropod abundances remain relatively stable through time. However, there is an apparent increase in gastropod genera which are common in seagrass environments which coincides with a decrease in genera common on hard substrate (i.e. coral). The transition from epifaunal to infaunal bivalves occurs in tandem with a decline in the abundance of large (>2mm) coral fragments in the cores; this shift, together with the change in gastropod assemblages, suggests a loss of coral and/or declines in coral growth in this region of Belize since the last half of the 19th century. These trends mirror those that have been previously observed from reef matrix cores from Caribbean Panama, suggesting region-wide declines in reef environmental conditions and ecosystems during the Late Holocene.