DECREASED TROPHIC COMPLEXITY OF MODERN CORAL REEF FOOD WEBS: NITROGEN ISOTOPIC EVIDENCE FROM FISH AND CORAL SKELETAL FOSSILS

Coral reefs are famed for their complex energetic pathways, which have been predicted to be strongly influenced by human impacts such as eutrophication, overfishing, and climate change. Yet we have no direct evidence of how trophic complexity has changed since the onset of human impacts. Here, we use the nitrogen isotopic composition (δ¹⁵N) of the organic constituents of fossil corals and otoliths to provide reconstructions of trophic pathways on fossil coral reefs and compare them to their modern counterparts. We focus on Caribbean coral reef systems in Panama and the Dominican Republic where large, exceptionally preserved mid-Holocene (7 ka) reefs are exposed. δ¹⁵N from corals track changes in the autotrophic baseline (δ¹⁵N_base) while δ¹⁵N from otoliths (δ¹⁵N_oto) provide a geochemical estimate for trophic level of fishes. In the Dominican Republic, δ¹⁵N_base from corals decreased from the mid-Holocene to today, consistent with previously observed changes in the nitrogen cycle over this time period (in particular, a basin-wide increase in biological nitrogen fixation since the last glacial period), demonstrating that our samples captured pre-human to modern end-members. In Panama, there was no significant change in the mean δ¹⁵N_base, suggesting that the basin-scale averages have been overprinted by the effects of increased δ¹⁵N from well-documented eutrophication in the region. Estimated trophic levels of Haemulidae and Apogonidae fishes from δ¹⁵N_oto values decreased in the DR, even when fish size and isotopic baseline conditions were accounted for, suggesting a reduction in trophic length in these ecosystems today. Mean δ¹⁵N_oto in most groups of fishes in Panama also remained similar over time, implying that trophic lengths are roughly similar on average. However, we observed a dramatic reduction in intra-taxon δ¹⁵N_oto variability in modern fishes compared to the mid-Holocene; a pattern that was also seen in the Dominican Republic. These findings suggest that energy flow pathways in modern reefs have become homogenised. The causes and consequences of this observation are discussed.