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CRES: Component: Demographic Studies: Coral Reef Communities
OVERVIEW
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Coral reefs have been well documented to be ecosystems of high biological diversity. Because substratum space is generally regarded as the primary limiting resource for sessile reef organisms, biological characterization of local reef systems will concentrate on the provision, acquisition, and retention of space. Natural and anthropogenic stresses are expected to be manifested through space-related processes. For example, the provision of space following coral mortalities may reflect direct effects of stresses (hurricanes, disease, bleaching and sedimentation) or biological interactions (predation and competition). In addition, other stresses as nutrient inputs may affect biological interactions (e.g., competition between algae and corals).
The benthic biology component of the CRES program involves high resolution examination of the stability of the coral community structure and demography (recruitment, growth, survivorship, etc.) of major coral reef organisms. These studies are currently being conducted at La Parguera, Puerto Rico, on the southwest coast near the Magueyes Island laboratory of the UPR Department of Marine Sciences. Fish, macroalgal and macroinvertebrate (principally including corals, gorgonians and sea urchins) populations are all being studied at the same transects. Eight reef sites: three near shore, three mid-shelf and two shelf edge reefs were selected and permanent transects at four depths (per reef site) with permanent quadrats have been established. Community structure will be assessed with repeated surveys of the same permanent transect/quadrats, thus allowing a documentation of change. Photo-quadrats along the permanent transect lines at each reef locality will be conducted to support the quantitative surveys. These field sites include reefs within and outside a proposed local marine reserve. Although other studies have been conducted previously in Puerto Rico, this represents the first time that all major components of coral reef communities will be examined in the same locations simultaneously. It is expected that these studies will provide new insights into the dynamics of coral reef communities.
The purpose of these studies is to characterize the distribution, cover and abundance of reef organisms within the study area, understand their population dynamics and detect spatial and temporal changes in reef composition and live cover. Comparison of the biology of reef systems as a function of position on the insular shelf, will provide insight into watershed effects. It is expected that by assessing possible changes in terms of the physical and chemical information gathered in CRES studies, we will be able to identify responsible stress factors.
Diadema Transplant Experiment: An underlying theme of the CRES demographic studies are the direct and indirect linkages between various biological, physical and chemical processes on coral reefs. The importance of certain coral reef organisms may be highly complex. We will study biological effects of one of these, the long spined sea urchin Diadema antillarum, which can be regarded as a keystone grazer based on its ecology and consequences of its mass mortality. Diadema grazes on turf and macroalgae and in the process it can ingest coral tissue and other sessile recruits; thus, it may be beneficial for larger corals via the removal of macroalgae but unfavorable for (sexual) coral recruitment when the densities of this urchins are high. Diadema is also a major bioeroder, producing 'clean', easily transportable sediments that can smother recruit colonies. Thus, an increase in gorgonian recruitment occurring shortly after the Diadema mass mortality was evidently due to decreased grazing as well as the decreased production of sediments and its consolidation by microflora. We will monitor an experimental site for 1 y prior to transplanting Diadema. In experimental sites, high density of urchins will be maintained for 1 y and then, urchins will be removed. The subsequent recovery processes will then be followed for 2 y. Sediment transport regimes as well as algal, coral, gorgonian and fish populations will be monitored during the entire period. We expect higher recruitment of corals and other sessile invertebrates in the experimental areas following the removal of Diadema, with a gradual (2 y) decline in recruitment due to changes in the sediment transport regime.
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