Parrotfish predation on massive Porites on the Great Barrier Reef

Parrotfish grazing scars on coral colonies were quantified across four reef zones at Lizard Island, Northern Great Barrier Reef (GBR). The abundance of parrotfish grazing scars was highest on reef flat and crest, with massive Porites spp. colonies having more parrotfish grazing scars than all other coral species combined. Massive Porites was the only coral type positively selected for grazing by parrotfishes in all four reef zones. The density of parrotfish grazing scars on massive Porites spp., and the rate of new scar formation, was highest on the reef crest and flat, reflecting the lower massive Porites cover and higher parrotfish abundance in these habitats. Overall, it appears that parrotfish predation pressure on corals could affect the abundance of preferred coral species, especially massive Porites spp, across the reef gradient. Parrotfish predation on corals may have a more important role on the GBR reefs than previously thought.     

3-D distribution of nongeniculate corallinales: a case study from a reef crest of South Sinai (Red Sea,Egypt)

An innovative technique for the estimation of species and growth-form abundance of coralline algae, including information on their vitality, was adopted on the reef crest of Ras Nosrani and Coral Bay, South Sinai. Data of coralline abundance from visual census and collection of voucher specimens were plotted on a 3-D sketched representation of the horizontal and vertical planes of the reef crest and of its crannies. Coralline dominance at the two investigated sites was not significantly different, with values ranging from 8.55 and 10.06% on the vertical plane and from 5.3 to 7.17% on the horizontal plane. About one-third of total corallines of the South Sinai reef crest was located in crannies, where the algae are completely overlooked by routine field surveys. Pink to violet, healthy corallines with encrusting growth-form, mainly belonging to Hydrolithon onkodes and Neogoniolithon, with subordinate fruticose Lithophyllum kotschyanum dominated the reef crest at both sites. The fruticose growth form, usually associated with L. kotschyanum, was more common in the horizontal than in the vertical plane. Purple, healthy, encrusting Sporolithon uncommonly occurred. Whitish, gray or green unhealthy or dead corallines were more common on the horizontal plane at both sites, possibly resulting from excessive solar radiation.     

Grazing effects of the periwinkle <Emphasis Type="Italic">Echinolittorina</Emphasis><Emphasis Type="Italic">peruviana</Emphasis> at a central Peruvian high rocky intertidal

Echinolittorina peruviana is the most common gastropod in the high intertidal zone of Peru, representing more than 80% of the individuals present at that zone. Experimental removal of snails was used to evaluate their effects on (a) abundance of epilithic biofilm, (b) barnacle recruitment, and (c) abundance of macroalgae under “normal” conditions of the El Niño Southern Oscillation (ENSO). Experiments were carried out from October 2005 to April 2007 at two intertidal levels of a semi-protected rocky shore of central Peru. Results demonstrated that E. peruviana is able to control biofilm abundance and barnacle recruitment at both heights investigated, with marked effects in the lower zone. Erect macroalgae (Ulva spp. and Gelidium spp.) were less affected by grazing; but negative effects were observed on macroalgal crusts. Season and physical stress seem to play a more important role in the abundance of macroalgae in the high intertidal. Our results are similar to those reported elsewhere for high shore littorinids and represent baseline data to understand how the role of intertidal consumers will vary under the cold (La Niña) and warm (El Niño) phases of ENSO on these shores.     

The aftermath of coral bleaching on a Maldivian reef—a quantitative study

Since the bleaching event of 1998, the development of the reef flat and upper reef slope on a Maldivian reef (the Komandoo house reef; Komandoo Island, Lhaviyani atoll or Fadiffolu atoll) is under detailed observation. We quantitatively recorded specific losses, re-colonisation by coral larvae on transects on the reef flat and on dead Acropora tables at the reef slope and regeneration of partly damaged large Porites and Diploastrea—colonies over the period from 1999 to 2004. The detrimental effects on the reef structure by bioerosion and hydrodynamics, as well as the overall status of the reef community were qualitatively assessed. Recruitment soon after the bleaching was more pronounced than in the following years, Pavona varians being a main constituent. The temporal re-colonisation pattern points at an emergency spawning of local Scleractinia just prior to the bleaching, whereas a sharp decrease of young settlers in 2001 and 2002 confirms a reduction of fertile colonies. The dominant species in the coral community shifted from acroporids and pocilloporids to agariciids. The skeletal deposition of recovering Diploastrea heliopora was equivalent to that before the bleaching, but much less than that of neighbouring Porites lobata colonies. The slow and scattered formation of new reef substance, which would structurally strengthen the reef, is however outweighed by the collapse of dead protruding and spacious colonies (e.g., Acropora tables). Six years after the bleaching, the formerly three-dimensional structure of the reef flat and upper reef slope presents as a levelled field of rubble, only partly consolidated by incrusting corals. Considering the recurrence of bleaching events (1987, 1998) and the results of the present study, one may assume a cascading deterioration of the status of the reef for the future.     

Reef demise and back-stepping during the last interglacial, northeast Yucatan

The elevation of reefs and coastal deposits during the last Interglaciation (MIS-5e) indicates that sea level reached a highstand of as much as 6 m above the present, but it is uncertain how rapidly this level was attained and how it impacted reef development. To investigate this problem, I made a detailed sedimentological analysis of a well-dated reef from the northeast coast of the stable Yucatan Peninsula. Two linear reef tracts were delineated which are offset and at different elevations. The lower reef tract crops out along northern shore for 575 m and extends from below present mean sea level to +3 m. The reef crest facies consists of large Acropora palmata colonies dispersed within a coral boulder-gravel and is flanked by an A. cervicornis-dominated reef-front and a large area of lagoonal framework formed by coalesced patches of A. cervicornis and Montastraea spp. Constituents in the upper centimetre of the lower tract are heavily encrusted by a cap of crustose corallines and, in places, are levelled by a discontinuous marine-erosion surface. The upper reef tract crops out ~150 m inland up to an elevation of +5.8 m and parallels the southern section of shore for ~400 m. It also consist of an A. palmata-dominated crest facies flanked by reef-front, back-reef and lagoonal frameworks. In this case, however, lagoonal frameworks are dominated by a sediment-tolerant assemblage of branching coralline algae. Also different is the lack of encrustation by corallines, and the infiltration of upper tract facies by beach-derived shell-gravels from regressive shoreface deposits above. These results indicate that the lower reef tract and lagoonal patch-reefs formed at a sea level of +3 m. Final capping by crustose corallines and discontinuous marine erosion indicates that the lower tract was terminated by the complete demise of corals on the crest but only patchy demise in the lagoon. Areas of continuous framework accretion between the lagoonal patch reefs and the upper reef-tract, however, require that the demise of this reef was ecologically synchronous with initiation of the upper reef-tract, which had back-stepped 100 m into the lagoon. In this new position, the upper tract developed a reef crest that corresponded to a final sea-level position of +6 m. Reef flat development at +5 m and large in-place colonies of A. palmata at the base of the crest unit indicate, however, that sea level must have risen rapidly from +3 to more than +5 m to accommodate back-stepping. This sea-level jump created a higher energy wave field that mobilized back-reef and lagoonal sediments, and the resulting high sediment flux eroded lagoonal framework and prevented the recovery of the submerged lower reef crest. So this single jump in sea level was responsible not only for reef demise and back-stepping but also for marine erosion and suppression of subsequent reef development—features that elsewhere have been used to support multiple sea-level excursions during the last interglacial.     

Population status of giant clams (Mollusca: Tridacnidae) in the northern Red Sea,Egypt

Throughout their range, giant clams (family Tridacnidae) are increasingly threatened by anthropogenic impacts and natural disasters, but little is known about their population status. In this first assessment of the tridacnid population at Abu Sauatir in the northern Red Sea, a total of 491 m² were surveyed and >200 clams recorded. Tridacna maxima was the only species found. The population's live:dead ratio was 3:1. Overall clam density was 0.08?±?0.008 live and 0.02?±?0.007 dead individuals per 0.25 m². Greatest densities occurred on the reef flat in 1 m depth (live), and on the northern reef slope in deeper waters (dead). On the slope, live clam density decreased significantly, whereas dead clam density increased significantly with depth. Sizes of live and dead individuals differed significantly. Live clams ranged from 1 to 30 cm (median 5 cm). Juveniles ≤2 cm (8.2% of the population) and individuals >11 cm occurred on the reef slope but not on the reef flat. Live clam sizes did not differ significantly between reef sites. Dead clam size ranged from 2 to 15 cm (median 6.5 cm). 2.1% of the empty shells were ≤2 cm long. Dead clam sizes differed significantly between 5 and 10 m depth on the northern reef slope. The low clam abundance (live and dead) in the shallowest and most easily accessible areas of the reef flat, combined with small sizes, strongly suggest artisanal reef-top gathering for meat and shells.     

Interactions Between Habitat Use and Patterns of Abundance in Coral-dwelling Fishes of the Genus <Emphasis Type="Italic">Gobiodon</Emphasis>

Coral-dwelling fishes from the genus Gobiodon are some of the most habitat specialised fishes on coral reefs. Consequently, we might expect that their population dynamics will be closely associated with the abundance of host corals. I used a combination of log-linear modelling and resource selection ratios to examine patterns of habitat use among eight species of Gobiodon in Kimbe Bay, Papua New Guinea. I then used multiple regression analysis to investigate relationships between the abundance of each species of Gobiodon and the abundance of the corals they inhabited. Each species of Gobiodon used one or more species of coral more frequently than expected by chance. The pattern of habitat use exhibited by each species of Gobiodon did not vary among reef zones or among reefs with different exposures to prevailing winds, despite changes in the relative abundances of corals among reef zones. This consistency in habitat use might be expected if the coral species inhabited confer considerable fitness advantages and, therefore, are strongly preferred. For most species of Gobiodon, abundances among reef zones and exposure regimes were correlated with the abundance of the coral species usually inhabited. Therefore, it appears that habitat availability helps determine abundances of most species of Gobiodon in Kimbe Bay. In addition to correlations with habitat availability, the abundances of G. histrio, G. quinquestrigatus, G. rivulatus (dark form) and the group others were also associated with particular reef zones and exposure regimes. Therefore, in these species, reef type appears to influence patterns of abundance independently of coral availability. In contrast to other species of Gobiodon, the abundance of the most specialised species, Gobiodon sp.A, was not closely associated with the abundance of the only coral species it inhabited. This study demonstrates that even for habitat specialised species, the relationship between habitat availability and abundance varies widely and is multiscale.     

Temporal variations of macroborers in massive Porites lobata on Moorea,French Polynesia

Massive colonies of Porites lobata on the barrier reef of Tiahura, Moorea, can be divided into four categories: living colonies, colonies consisting of 50% live coral and 50% dead skeleton, 100% dead coral and colonies which have been reduced to a basal plate. Replicate samples of each of these colony types were collected in the same vicinity of the barrier reef during October 1987. The macroborers were extracted, identified, counted and their volumes determined by displacement. Kruskal-Wallis tests showed that three different boring communities occur within these four categories of Porites colonies. Live colonies are characterised by only 3 species, the bivalve, Lithophaga laevigata; the vermetid Dendropoma maximun; and the non boring serpulid polychaete Spirobranchus. The completely dead colonies contain up to 17 boring species, with five to six individuals per 100 cm. Sipunculans are the dominant bioeroders with the most abundant species being Aspidosiphon elegans, sp A and sp B. Colonies of Porites which have been reduced to basal plates contain up to 18 boring species of which the bivalve Lithophaga hanleyana and the sipunculan Aspidosiphon sp. B are the most abundant.The cumulative volume of CaCO₃ lost by boring activity increases from 0.1 cm³ per 100 cm³ in a completely dead Porites colony to 1.4 cm³ per 100 cm³ in the residual basal plates of Porites. These can be extrapolated to minimum losses of 14.2 kg m^-3. We suggest that rates of boring increase with the time which has elapsed since the death of the colony and the dominant agents of boring also change with increasing age of the coral structure. There are significant additional losses of 5.25 kg m^-3 CaCO₃ caused by grazing echinoids and scarids.     

Territoriality and Habitat Use by Juvenile Blue Tangs, Acanthurus coeruleus

We studied territoriality and habitat use by yellow phase juvenile blue tangs, Acanthurus coeruleus, on a small fringing reef in Barbados, West Indies. Juvenile blue tangs occurred on the reef crest, spurs, and a transition zone between the reef crest and reef flat at a density of about 8 individuals per 100m², but were much rarer on the reef flat. They were solitary and occupied stable home ranges (median=0.85m²) that increased with body size. Observational and experimental data documented aggressive defense of home ranges against conspecific and to a lesser extent congeneric, A. bahianus, juveniles (about 7.5 approaches and attacks per hour directed at intruders). Home range locations were structurally more complex and closer to a vertical face than expected by chance. Although juvenile blue tang territories overlapped considerably with those of larger and more aggressive Stegastes damselfish, which are believed to exclude solitary adult Acanthurus spp. from reef crest and spurs, the tangs avoided Stegastes and were rarely chased (<0.3 fleeing events per hour). Space use and social organization of yellow juvenile blue tangs contrast strikingly with that of both conspecific adults and congeneric juveniles.     

The dynamics of benthic microbial communities at Davies Reef,central Great Barrier Reef

The dynamics of benthic microbial communities were examined within different functional zones (reef crest, reef flat, lagoon) of Davies Reef, central Great Barrier Reef, in winter. Bacterial numbers did not change significantly across the reef with a mean abundance of 1.3 (±0.6) x 10⁹ cells g^-1 DW of sediment. Bacterial production, measured as thymidine incorporation into DNA, ranged from 1.2 (±0.2) to 11.6 (±1.5) mg C m^-2h^-1 across the reef and was significantly lower in a reef crest basin than in the other zones. Bacterial growth rates () across the reef (0.05 to 0.33 g^-1) correlated only with sediment organic carbon and nitrogen. Protozoan and meiofaunal densities varied by an order of magnitude across the reef and correlated with one or more sediment variables but not with bacterial numbers or growth rates. Nutrient flux rates were similar to those found at other reefs in the central and southern Great Barrier Reef and are significantly lower than rates measured in temperate sand communities. In the front lagoon, bioturbation and feeding acitivity by thalassinid shrimps (Callianassa spp.) negatively influenced microbial and meiofaunal communities with a net import of organic matter necessary to support the estimated rates of bacterial productivity. In lagoonal areas not colonized by shrimps, primary productivity (400–1100 mg C m^-2d^-1) from algal mats was sufficient to support bacterial growth. It is suggested that deposit-feeding macrobenthos such as thalassinid crustaceans play a major role in the tructuring and functioning of lower trophic groups (bacteria, microalgae, protozoa, meiofauna) in coral reef sedments, particularly in laggons.     

Partial Mortality in Porites Corals: Variation among Philippine Reefs

Partial mortality or tissue necrosis was quantified in the massive scleractinian coral Porites at three sites in The Philippines (Bolinao, NW Luzon; Puerto Galera, Mindoro; and El Nido, N Palawan). Overall, 15 ± 1 (mean ± 1 standard error, 642 replicates) percent of colony area was dead, mean colony area was 1135 plusmn; 127 cm², and lesion density was 1.7 ± 0.1 dm^—2. Total live coral cover varied between 20 and 63% in belt transects, and Porites and Acropora cover were inversely correlated. ANOVA models incorporating effects of site, colony size, sedimentation rates, wave exposure and depth were highly significant but explained only a small proportion of the variation observed in lesion density and percent dead area (respectively 8 and 2%). Lesion density was found to vary significantly with site (contributed 29% to this explained variance), decrease with increasing colony area (33%), and increase with increasing sedimentation (23%) and wave exposure (14%). Colony size was significantly explained by the factor site (contributing 61% to the total 29% explained variance) and depth (34%), with the smallest colonies being observed in Bolinao and the largest in El Nido. Densities of lesions were highest in Bolinao, intermediate in Puerto Galera, and lowest in El Nido. This pattern is parallel to intensity of human reef exploitation and opposite to that in colony size, live coral cover and Acropora cover. Since only a small part of the observed variance in partial mortality estimators was explained by the ANOVAs, other factors not quantified here must have been more important (e.g. disease incidence, predation, human exploitation).     

Growth of reef fishes in response to live coral cover

Although the global decline in coral reef health is likely to have profound effects on reef associated fishes, these effects are poorly understood. While declining coral cover can reduce the abundance of reef fishes through direct effects on recruitment and/or mortality, recent evidence suggests that individuals may survive in disturbed habitats, but may experience sublethal reductions in their condition. This study examined the response of 2 coral associated damselfishes (Pomacentridae), Chrysiptera parasema and Dascyllus melanurus, to varying levels of live coral cover. Growth, persistence, and the condition of individuals were quantified on replicate coral colonies in 3 coral treatments: 100% live coral (control), 50% live coral (partial) and 0% live coral (dead). The growth rates of both species were directly related to the percentage live coral cover, with individuals associated with dead corals exhibiting the slowest growth, and highest growth on control corals. Such differences in individual growth between treatments were apparent after 29 d. There was no significant difference in the numbers of fishes persisting or the physiological condition of individuals between different treatments on this time-scale. Slower growth in disturbed habitats will delay the onset of maturity, reduce lifetime fecundity and increase individual's vulnerability to gape-limited predation. Hence, immediate effects on recruitment and survival may underestimate the longer-term impacts of declining coral on the structure and diversity of coral-associated reef fish communities.     

Distribution of a nematocyst-bearing sponge in relation to potential coral donors

Haliclona sp. 628 (Demospongiae, Haplosclerida, Chalinidae), a sponge found on the reef slope below 5 m depth on the Great Barrier Reef, has two unusual characteristics. It contains a symbiotic dinoflagellate, Symbiodinium sp., similar in structure to the dinoflagellate found within Acropora nobilis (S. microadriaticum), and it contains coral nematocysts randomly distributed between the ectosome and endosome and usually undischarged in intact sponge tissue. Given the unusual occurrence of nematocysts in Haliclona sp. 628, the focus of this study was to determine the distribution of this species of sponge on the reef slope at Heron Island Reef in relation to the distribution of potential coral donors. A combination of line and belt transects was used to estimate the abundance of Haliclona sp. 628 and a co-occurring congener, Haliclona sp. 1031, which does not contain nematocysts, at three widely separated sites on the reef slope at Heron Island Reef. The abundance of different types of substratum (sand, sand-covered coral rubble, dead A. nobilis, live A. nobilis, other live coral, and other dead coral) along the transects and the substratum to which each sponge colony was attached were also recorded. Despite the predominance of live A. nobilis and sand-covered rubble at all sites, between 30 and 55% of Haliclona sp. 628 colonies were attached to dead A. nobilis which comprised less than 8% of the available substratum along any transect. In contrast, Haliclona sp. 1031 was found significantly more frequently on other dead corals and less frequently on live A. nobilis than would be expected based on the availability of the different substrata in the sites. Potential explanations to account for the distribution of Haliclona sp. 628 in relation to potential coral donors are discussed.     

Habitat choice,recruitment and the response of coral reef fishes to coral degradation

The global degradation of coral reefs is having profound effects on the structure and species richness of associated reef fish assemblages. Historically, variation in the composition of fish communities has largely been attributed to factors affecting settlement of reef fish larvae. However, the mechanisms that determine how fish settlers respond to different stages of coral stress and the extent of coral loss on fish settlement are poorly understood. Here, we examined the effects of habitat degradation on fish settlement using a two-stage experimental approach. First, we employed laboratory choice experiments to test how settlers responded to early and terminal stages of coral degradation. We then quantified the settlement response of the whole reef fish assemblage in a field perturbation experiment. The laboratory choice experiments tested how juveniles from nine common Indo-Pacific fishes chose among live colonies, partially degraded colonies, and dead colonies with recent algal growth. Many species did not distinguish between live and partially degraded colonies, suggesting settlement patterns are resilient to the early stages of declining coral health. Several species preferred live or degraded corals, and none preferred to associate with dead, algal-covered colonies. In the field experiment, fish recruitment to coral colonies was monitored before and after the introduction of a coral predator (the crown-of-thorns starfish) and compared with undisturbed control colonies. Starfish reduced live coral cover by 95–100%, causing persistent negative effects on the recruitment of coral-associated fishes. Rapid reductions in new recruit abundance, greater numbers of unoccupied colonies and a shift in the recruit community structure from one dominated by coral-associated fishes before degradation to one predominantly composed of algal-associated fish species were observed. Our results suggest that while resistant to coral stress, coral death alters the process of replenishment of coral reef fish communities.     

Coral mortality versus structural collapse as drivers of corallivorous butterflyfish decline

As climate change increasingly threatens biodiversity, identifying specific drivers of species loss as well as the attributes of species most vulnerable to climatic disturbances is a key challenge to ecologists and conservationists. Here we assess the effects of coral loss versus declines in structural complexity on obligate and facultative coral feeding butterflyfishes on coral reefs in the central and western Indian Ocean. In the inner Seychelles, the abundance of the obligate coral feeding group declined markedly in response to live coral mortality (r ² = 0.48), but showed no further decline with respect to erosion of the physical matrix of the reef. Conversely, the facultative feeding group showed no decline in response to live coral loss, reflecting their feeding versatility; however they did decline in response to structural erosion of the reef framework (r ² = 0.26). There were no significant changes in either obligate or facultative corallivore abundances at a reference location (Chagos archipelago), highlighting that butterflyfish populations are stable in the absence of habitat disturbance. While specialised coral dependant fishes are highly vulnerable to coral loss caused by climate-induced coral bleaching, the structural collapse of dead coral colonies may have significant, but more variable, impacts across a wide range of fishes. If conservation and mitigation planning are to be effective, there is a clear need to better understand the mechanisms of reef structural collapse and the dynamics of system recovery following large-scale disturbance.     

Influence of habitat degradation on fish replenishment

Temperature-induced coral bleaching is a major threat to the biodiversity of coral reef ecosystems. While reductions in species diversity and abundance of fish communities have been documented following coral bleaching, the mechanisms that underlie these changes are poorly understood. The present study examined the impacts of coral bleaching on the early life-history processes of coral reef fishes. Daily monitoring of fish settlement patterns found that ten times as many fish settled to healthy coral than sub-lethally bleached coral. Species diversity of settling fishes was least on bleached coral and greatest on dead coral, with healthy coral having intermediate levels of diversity. Laboratory experiments using light-trap caught juveniles showed that different damselfish species chose among healthy, bleached and dead coral habitats using different combinations of visual and olfactory cues. The live coral specialist, Pomacentrus moluccensis, preferred live coral and avoided bleached and dead coral, using mostly visual cues to inform their habitat choice. The habitat generalist, Pomacentrus amboinensis, also preferred live coral and avoided bleached and dead coral but selected these habitats using both visual and olfactory cues. Trials with another habitat generalist, Dischistodus sp., suggested that vision played a significant role. A 20 days field experiment that manipulated densities of P. moluccensis on healthy and bleached coral heads found an influence of fish density on juvenile weight and growth, but no significant influence of habitat quality. These results suggests that coral bleaching will affect settlement patterns and species distributions by influencing the visual and olfactory cues that reef fish larvae use to make settlement choices. Furthermore, increased fish density within the remaining healthy coral habitats could play an important role in influencing population dynamics.     

Recruitment failure in Florida Keys <Emphasis Type="Italic">Acropora palmata,</Emphasis> a threatened Caribbean coral

Recovery of Acropora palmata from its currently imperiled status depends on recruitment, a process which is poorly documented in existing Caribbean coral population studies. A. palmata is thought to be well adapted to proliferate through the recruitment of fragments resulting from physical disturbances, such as moderate intensity hurricanes. This study monitored fifteen 150 m² fixed study plots on the upper Florida Keys fore-reef for asexual and sexual recruitment from 2004 to 2007. Between July and October 2005, 4 hurricanes passed by the Florida Keys, producing wind speeds on the reef tract of 23 to 33 m s⁻¹. Surveys following the hurricanes documented an average loss of 52% estimated live tissue area within the study plots. The percentage of “branching” colonies in the population decreased from 67% to 42% while “remnant” colonies (isolated patches of tissue on standing skeleton) increased from 11% to 27%. Although some detached branches remained as loose fragments, more than 70% of the 380 fragments observed in the study plots were dead or rapidly losing tissue 3 weeks after Hurricane Dennis. Over the course of the study, only 27 fragments became attached to the substrate to form successful asexual recruits. Meanwhile, of the 18 new, small encrusting colonies that were observed in the study, only 2 were not attributable to asexual origin (i.e., remnant tissue from colonies or fragments previously observed) and are therefore possible sexual recruits. In summary, the 2005 hurricane season resulted in substantial loss of A. palmata from the upper Florida Keys fore-reef from a combination of physical removal and subsequent disease-like tissue mortality, and yielded few recruits of either sexual or asexual origin. Furthermore, the asexual and sexual fecundity of the remaining population is compromised for the near future due to the lack of branches (i.e., “asexual fecundity”) and overall loss of live tissue.     

Population structure of an introduced species (Dreissena polymorpha) along a wave-swept disturbance gradient

The zebra mussel Dreissena polymorpha was introduced to North America during the mid-1980s, and is now a dominant member of many benthic communities in the lower Great Lakes. In this study, I explored the abundance, biomass, size structure and settlement of Dreissena inhabiting rocks along a wave-swept disturbance gradient near Middle Sister Island in western Lake Erie. Ten rocks were collected from quadrats at six sites along each of three transect lines oriented perpendicular to shore. Occurrence, abundance and biomass of Dreissena on smaller, movable rocks were positively associated with rock distance from shore (lake depth) and with rock area; rocks at nearshore sites supported little, if any, Dreissena, whereas those at offshore sites were heavily colonized. Mussel size distributions also differed in relation to shore distance. Large mussels (19 mm) were underrepresented or absent on rocks collected at nearshore sites, but were overrepresented at offshore locations (37 m). Settlement of larval mussels on settling pads was positively correlated with distance offshore and with time of exposure, though settlement was substantial even at a nearshore (10 m) location. Area-adjusted mussel dry mass increased more rapidly with distance offshore on large than on small rocks. Large rocks also required more force to displace and were significantly less likely to be disturbed when transplanted at the study site. Results from this study indicate that occurrence, abundance and size structure of Dreissena in nearshore waters of Lake Erie correspond with the frequency of habitat disturbance, though other factors including food limitation and larval supply may also contribute to these patterns. These patterns complement studies that established the significance of physical disturbance in other aquatic systems.