Artificial reefs

Artificial reefs are coastal structures built to improve marine life and prevent beach erosion. During earlier days artificial reefs were constructed for recreational fishing using discarded scraps and waste materials. Later on, ships were scuttled for constructing artificial reefs. Artificial reefs dissipate the energy of the wave by making the wave break over the reef. The artificial reefs used for coastal protection are usually in submerged condition as this condition does not affect the aesthetic beauty of the beach. Wave transmission decides the efficiency of submerged-detached artificial reef in protecting the beach from the incoming waves. The efficiency of submerged detached coastal protection structures in protecting the beach is usually measured in terms of wave transmission coefficient.
The experimental investigation in the present study is carried out for submerged two-dimensional impermeable and permeable reefs for three water depths. The crest width of the reefs considered for the experimental studies are 60 cm and 20 cm. The permeable artificial reefs are made up of oyster shells in Nylon bags and biodegradable bags. The water levels considered for the study are 35 cm, 34 cm, and 33 cm. The effect of pore space between the oyster shells, crest width, water depth and wave parameters on the wave transmission coefficient for submerged impermeable and permeable artificial reefs are studied experimentally. The wave transmission coefficient is calculated for submerged impermeable and permeable reefs for different water levels and crest widths. Based on the results of the present experimental studies, it is logical to conclude that both submerged impermeable and permeable artificial reefs contribute to a significant extent to the attenuation of the incident wave.

The effect of artificial reefs on natural reefs is poorly understood. This study focused on Aquarius Reef Base (ARB), an underwater habitat offshore of Key Largo, Florida, and 14 natural reef sites spanning 4 habitats, on the surrounding Conch Reef. Food web models were created for ARB and natural reef habitats. Biomass at each habitat was quantified by fish surveys. Using Ecopath, species were organized into functional groups with supporting data from previous research for other inputs. ARB’s food web was found to have a large predator biomass with insufficient prey biomass to sustain the population, suggesting that these predators must forage on nearby natural reefs where the predator/prey ratio is smaller. Between 0.57km2 and 1.79km2 of natural reef is estimated to be a sufficient spatial subsidy for the large predatory biomass at ARB when the biomass is added as determined by the seascape around the artificial reef.

Rock and coral reef fishes are known to select their habitat, and selection oftentimes involves the settlement of pelagic larval stages. Thus, I examined the short-term temporal relationship between artificial reef morphology and the composition of warm-water reef fish assemblages 30 m offshore of the Town of Palm Beach, Florida. Correlations were found between reef height and total fish abundance and species richness. Reef height was also correlated with log transformed numbers of juveniles. Horizontal opening size showed an inverse correlation with species richness and a weak inverse trend with juvenile abundance. Furthermore, vertical openings were directly proportional to juvenile and total fish abundance, and to species richness. Piscivore abundance was weakly influenced by presence and dimensions of vertical crevices. Thus, artificial reefs with many varied-sized vertical crevices are most desirable for attracting juvenile warm-water reef fishes.

The Boca Raton artificial reef was designed to mitigate for the burial of nearshore rock habitat resulting from beach restoration. Seasonal recruitment and community development of the reef were studied from April 1988 to July 1990. Species diversity within the fish community reached an equilibrium 2-2.5 months following reef placement. Seasonality was recorded for juvenile, intermediate and adult life history stages. Encrusting invertebrates and algae exhibited preferences for vertical and horizontal substrates, respectively, while nonencrusting invertebrates exhibited no preference for surface orientation. No conspicuous seasonal trends were evident for the encrusting or non-encrusting invertebrates, except for Watersipora sp. and Botryllus schlosseri. Equilibrium was not attained by invertebrates or algae, nor was there evidence of true ecological succession beyond the earliest stages. Instead, the reef appeared to be maintained in an early stage of development as a result of frequent physical disturbance.