The effects of nitrogen and phosphorus enrichment on algal community development: Artificial mini-reefs on the Belize Barrier Reef sedimentary lagoon

The experiments to compare DIN and SRP enrichment effects on algal community development were conducted within a lagoonal rubble/sand reef apron west of the back reef flat at Carrie Bow Cay, Belize. Macroalgae dominate (23 taxa, 57% cover), ambient dissolved inorganic nutrient concentrations are above the levels documented for release of macroalgal growth (means of 2.06 mu M DIN = dissolved inorganic nitrogen and 0.12 mu M SRP = soluble reactive phosphorus) and grazing is negligible (1.7% Acanthophora spicifera consumed 6 h(-1)). Mini-reef diffusers containing slow-release fertilizers significantly (P < 0.05) increased DIN in the experimental DIN and SRP + DIN treatments by 2- and 3-fold (means of 4.64 and 6.41 mu M), respectively: while SRP was increased significantly (P < 0.05) in the SRP and SRP + DIN treatments by 5- and 8-fold to means of 0.69 and 0.94 mu M, relative to the control treatments. SRP and SRP + DIN treatments favored colonization by Cyanobacteria such as Spirulina sp. on the mini-reef/diffusers, with an opposite detrimental effect on reef-building crustose coralline algae. The Cyanobacteria bloom further inhibited the long-term settlement and colonization of crustose coralline algae. Conversely, the DIN and control mini-reef treatments showed low Cyanobacteria cover and became colonized by abundant reef-building coralline algae, consisting mostly of Hydrolithon boergesenii and crust stages of Amphiroa fragilissima. After 1 year, the competitively overgrowing macrophytes A. spicifera, Palisada papillosa, Padina sanctae-crucis and Spyridia filamentosa conspicuously dominated all of the treatment- and control-replicates, in accordance with the nutrient-replete ambient waters and negligible herbivory within this habitat. As predicted, nutrient additions to the mini-reef diffusers resulted in significant elevations of tissue nutrients in the most-abundant colonizer and habitat dominant, A. spicifera. Although, the resultant decreases of both the C:N and C:P molar ratios corresponded to the elevated DIN and SRP treatments, this uptake was entirely superfluous, since control population colonization and growth matched that of the experimental nutrient treatments. Published by Elsevier B.V.