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 μM DIN = dissolved inorganic nitrogen and 0.12 μM 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 μM), 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 μM, 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 Hydrolithonboergesenii and crust stages of Amphiroafragilissima. After 1 year, the competitively overgrowing macrophytes A.spicifera, Palisadapapillosa, Padinasanctae-crucis and Spyridiafilamentosa 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.