Digital library

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.

The observed high biomass of Sargassum species and other fleshy macroalgae on nearshore coral reefs in the Great Barrier Reef (Australia) raises the concern that this may be a sign of a phase shift from hard coral to dominance by algae, thus indicating the degradation of these reefs. Nearshore reefs are, due to their geographical position, exposed to inputs of nutrients and particles from mainland run-off. This study estimated the limiting and optimum nutrient concentrations for the growth of Sargassum baccularia under continuous supply of ammonium and phosphate. To assess the nutrient situation in the field, critical and subsistence levels of tissue nutrients as determined in cultures were compared with field tissue nutrient levels, and a 'mini budget' was estimated that compares nutrient requirements with nutrient supply. The growth rates of S. baccularia almost doubled within the narrow window of substrate concentrations from 3 mu M ammonium plus 0.3 mu M phosphate to 5 mu M ammonium plus 0.5 mu M phosphate. Lower and, unexpectedly, higher nutrient concentrations resulted in reduced growth rates. Field thalli of S. baccularia were always sufficiently supplied with N and P to show positive growth rates. However, field growth was both N- and P-limited, particularly in austral summer, when fast growth of S. baccularia occurs. This was indicated by tissue N and P concentrations that fell below the respective critical nutrient values, and estimated nutrient demands that exceeded the nutrients available from the water column. We suggest that if land-derived nutrient inputs increased, S. baccularia would become nutrient-sufficient, especially during the summer wet season. This may increase the competitive potential of this species to colonise larger areas on the nearshore reefs.

In terrestrial plants, it is well known that genetic diversity can affect responses to abiotic and biotic stress and have important consequences on farming. However, very little is known about the interactive effects of genetic and environmental factors on seaweed crops. We conducted a field experiment on Gracilaria chilensis to determine the effect of heterozygosity and nutrient addition on two southern Chilean farms: Ancud and Chaica. In addition to growth rate and productivity, we measured photosynthetic responses, photosynthetic pigment concentration (chlorophyll a and phycobiliproteins), C:N ratio (C:N), and epiphytic load. Nutrient addition affected the growth rate, productivity, phycobilin and C:N content, but not the epiphytic load. These results were independent of the heterozygosity of the strains used in the experiments. Interestingly, depending on the sampled sites, distinct photosynthetic responses (i.e., maximal quantum yield, Fv/Fm and maximal electron transport rate, ETRmax) to nutrient addition were observed. We propose that thallus selection over the past few decades may have led to ecological differentiation between Gracilaria chilensis from Chaica and Ancud. The lack of effect of heterozygosity on growth and physiological responses could be related to the species domestication history in which there is a limited range of genetic variation in farms. We suggest that the existing levels of heterozygosity among our thalli is not sufficient to detect any significant effect of genetic diversity on growth or productivity in Metri bay, our experimental site located close to the city of Puerto Montt, during summer under nitrogen limiting conditions.