Coral – algal interactions
MSEL have investigated the competitive effects of macroalgae (Dictyota pulchella, Lobophora variegata, Halimeda opuntia and mixed algal communities) on coral growth, survival and fecundity. Fecundity was studied on colonies of the coral Montastraea annularis, while growth and survival were studied in Colpophyllia natans, Porites astreoides and Agaricia agaricites. We experimentally manipulated algal contact around the perimeter of M. annularis patches for the entire period of gamete development. Fecundity was measured as the diameter of eggs, the number of eggs per gonad and the number of gonads per polyp. We found that algal contact significantly reduced the diameter of eggs at both the coral-algal boundary and at the centre of coral patches. The presence of Dictyota spp. and a mixed algal community were shown to have more detrimental effects on egg diameter than L. variegata. Removal of algal contact immediately prior to gametogenesis increased the reproductive output of polyps directly adjacent to the cleared areas. In terms of growth and survival corals smaller than 10 cm in diameter were significantly more vulnerable to macroalgal competition. Furthermore, H. opuntia proved to have a more detrimental effect on C. natans, while A. agaricites resulted to be the best competitor against both H. opuntia and L. variegata.
Algal population dynamics
The large-scale and long-term dynamics of benthic communities are poorly understood because the duration of experiments are rarely commensurate with the longevity of communities. However, processes causing long-term dynamics occur on different scales as reflected by changes in the spatial pattern of a community across the seascape. To identify these processes, we first create spatially-explicit dynamic models to understand how combinations of physical and biological processes affect the distribution of coral communities. Second, the output of each model is compared to natural patterns of community distribution as measured using high-resolution Earth Observation. Third, the models and significant processes are evaluated from a probabilistic comparison of plausible scenarios and by field survey of the age distribution of corals.
Members of MSEL have quantified the patch dynamics of three dominant species of macroalgae (Lobophora variegata, Dictyota pulchella and Halimeda opuntia) on a Caribbean fore reef in Belize. Using video quadrats collected between June 1998 and January 2003, colonization, growth and extinction rates were measured for Lobophora variegata and Dictyota pulchella. Measurements were taken on time scales of days, weeks, months, and years during which three hurricanes occurred. The first hurricane (Mitch) caused massive coral and algal mortality, liberating space for subsequent algal colonization. MSEL found that the dynamics of the two algal species differed markedly. Whilst the cover of D. pulchella varied considerably over time, the underlying patch dynamics were relatively stable. In contrast, L. variegata showed a steady increase in cover after hurricane Mitch, despite greater dynamism in the colonization and extinction of patches. We used further video quadrats collected between December 2008 and December 2009 to include H. opuntia in our research and improve our understanding of the dynamics of D. pulchella and L. variegata throughout the year. MSEL found that D. pulchella is strongly influenced by seasonal dynamics while L. variegata and H. opuntia strongly depend on herbivory, while also being affected by temperature variation. Thus, variation in the overall percent cover of an alga is not indicative of the underlying patch dynamics and it is key to consider the season when assessing macroalgae community structure and abundance. MSEL is also looking at the primary productivity of turf algae under different grazing and exposure levels. These data is a key component to understanding ecological processes that underlie macroalgal-coral interactions and phase shifts.
Sponges are well known because of the contribution to the functioning of coral reef ecosystems (i.e., bioerosion, competition, nutrient cycling, biodiversity, productivity). However, their role in the ecosystem may change as reef sustain greater levels of disturbance. Yet, how do sponges response to a number of perturbations and stressors, and moreover, how does this affect sponge-driven process in the ecosystem is poorly understood. MSEL is currently exploring at potential drivers on the structure of sponge population and communities (i.e., hurricanes, macroalgal competition and predation), looking at expanding our current knowledge on the ecosystem.
Coral recruitment is critical to coral reef communities as it represents a crucial phase in the development of coral populations, important to the recovery of coral reefs affected by disturbance and mortality. Successful recruitment is important to the resilience of coral reefs, and degraded reefs often exhibit declining rates of coral recruitment through a poorly understood combination of reduced adult fecundity, decreased settlement, and higher rates of early mortality. Coral recruitment is a complex process involving gamete production, fertilisation, dispersal and development, settlement, early survival and growth. Colonisation of the benthos by invertebrates involves the three phases of (1) larval development, which includes planktonic dispersal, (2) testing of the benthos for microhabitat suitability, and (3) larval settlement on the substrate. Following larval settlement, coral spat exist as minute individuals for a period of time during which they are very difficult to detect by the observer and high mortality rates notoriously occur. Following settlement, recruitment itself is defined as the number of individuals passing from the settled phase to the stage when new members of the community become visible to be censused, after which time the individuals are commonly referred to as juveniles.