Nutrient and growth dynamics of Halimeda tuna on Conch Reef, Florida Keys: Possible influence of internal tides on nutrient status and physiology

We conducted a manipulative nutrient enrichment study to examine the physiological and growth dynamics of the common tropical reef alga Halimeda tuna from shallow and deep coral reef environments on Conch Reef, in the Florida Keys. Paradoxically, H. tuna exhibited higher growth rates at depth (low light and below saturating irradiances) than at shallow sites (high light and at or above saturating irradiances). We hypothesized that the differences in growth rates were caused by differing nutrient environments at the two sites potentially caused by the influence of internal tidal bores that elevate nutrient concentrations with depth on Conch Reef. We tested this hypothesis by manipulating nutrients in a 10-d field experiment, after which we assessed growth, photosynthetic pigments, tissue nutrients, and other physiological parameters. H. tuna from the shallow back reef site exhibited nutrient limitation, as indicated by increases in growth rates, pigmentation, tissue nutrients, segment size, and photosynthetic rates, after enrichment. At the deep site, growth rates were not significantly different between controls and nutrient-enriched algae. Shallow enriched samples achieved levels of growth that were not significantly different from deep control or enriched samples. Algae from the deep site responded positively to enrichment for some physiological parameters; this suggests an opportunistic strategy in an environment that is known to experience frequent and significant pulses of nutrients from internal tides. Our results document differential nutrient limitation for H. tuna from two sites on Conch Reef where, in general, algae from the shallow site were more nutrient limited than those from the deep site. Finally, this provides some evidence that tropical reef communities may be adapted to large-scale physical processes such as internal tides.