Interactions Between Sponges and Macroalgae on Temperate Rocky Reefs

<p>Changes in the distributions of organisms not only alter community composition and food web structure, but also can initiate important changes at the ecosystem level. Understanding the interactions between biotic and abiotic factors affecting species’ distribution patterns in temperate habitats is important for predicting responses to future environmental change. Sponges are important members of temperate rocky reefs assemblages that are influenced by a number of abiotic factors including water movement, light regime, inclination and stability of the substratum, as well as complex ecological interactions. The aim of this thesis was to investigate the interactions between sponges and macroalgae on shallow-water rocky reefs of Wellington, New Zealand, assessing if the distribution patterns of sponges are independent of algal populations. I used a combination of surveys, and manipulative field and laboratory experiments to explore the existence of interactions (positive or negative) between sponges and macroalgae and also to explore the effect of environmental factors on the distribution and abundance of temperate sponges. My first objective was to determine if the spatial distribution patterns of sponges are independent of macroalgae distribution and abundance at different sites on the Wellington south coast (Chapter 2). The results showed that abundance of most sponge species were strongly correlated with inclination, which supports previous studies in the northern hemisphere suggesting that sponge abundance and algal abundance are negatively correlated. In contrast, only a few sponge species were positively correlated with algal abundance. I then explored the positive interactions occurring between some sponges species and the presence of canopy-forming algae (Chapter 3). Results from this chapter suggest the canopy of Ecklonia radiata facilitates the existence of some sponge species such as Crella incrustans on vertical rocky walls. The removal of Ecklonia canopy led to a community dominated by turf algae, which corresponded with a decrease in sponge abundance and richness. My results suggest that the Ecklonia canopy facilitates the presence of some sponge species and allows their coexistence with turf algae underneath the canopy and also by altering immediate physical factors that may be detrimental for some sponge species. To further explore the existence of sponges and understory algae, I used an experimental approach (Chapter 4) to investigate the effect of the brown alga Zonaria turneriana on Leucetta sp. and also mechanisms involved in the interactions. However results from this chapter provided no evidence to support previous hypotheses that understory algae negatively affect sponges. In the last data chapter (Chapter 5), I studied sponges inhabiting different habitats in order to test if environmental variation affects the abundance and diversity of microorganisms, hence having the potential to affect the distribution and abundance of these species The stability observed in bacterial communities among specimens occupying different habitats suggests that environmental variation occurring in those habitats does not affect the stability of the community, and hence most likely does not radically alter the metabolism of these sponges. Although environmental factors such as light and sediment may have an effect on early sponge stages, other environmental (e.g. nutrients, temperature, wave action) and biotic factors, are more likely to influence the growth, survival and distribution of sponges on temperate rocky reefs. In summary, temperate sponge assemblages are strongly influenced by interactions between a number of abiotic and biotic factors. The outcomes of the ecological interactions are controlled by environment (e.g. influence of inclination on competition between sponges and understory algae) and at the same time, biological interactions (e.g. facilitation) can moderate the influence of abiotic factors such as light, sedimentation and wave action, thus facilitating the coexistence between sponge and macroalgae underneath the Ecklonia canopy. My thesis makes a significant contribution to our knowledge of temperate subtidal ecology, in terms of the effects of biotic and abiotic factors on sponge assemblages and also improves our knowledge of temperate patterns of sponge and macroalgal interactions. Finally, my thesis highlights the importance of small-scale environmental variation in influencing the structure and diversity of sponge assemblages and also increase our understanding of temperate rocky reefs sponges, especially on the less studied sponge assemblages occurring in Ecklonia stands on vertical rocky walls.</p>

Interactions Between Sponges and Macroalgae on Temperate Rocky Reefs

<p>Changes in the distributions of organisms not only alter community composition and food web structure, but also can initiate important changes at the ecosystem level. Understanding the interactions between biotic and abiotic factors affecting species’ distribution patterns in temperate habitats is important for predicting responses to future environmental change. Sponges are important members of temperate rocky reefs assemblages that are influenced by a number of abiotic factors including water movement, light regime, inclination and stability of the substratum, as well as complex ecological interactions. The aim of this thesis was to investigate the interactions between sponges and macroalgae on shallow-water rocky reefs of Wellington, New Zealand, assessing if the distribution patterns of sponges are independent of algal populations. I used a combination of surveys, and manipulative field and laboratory experiments to explore the existence of interactions (positive or negative) between sponges and macroalgae and also to explore the effect of environmental factors on the distribution and abundance of temperate sponges. My first objective was to determine if the spatial distribution patterns of sponges are independent of macroalgae distribution and abundance at different sites on the Wellington south coast (Chapter 2). The results showed that abundance of most sponge species were strongly correlated with inclination, which supports previous studies in the northern hemisphere suggesting that sponge abundance and algal abundance are negatively correlated. In contrast, only a few sponge species were positively correlated with algal abundance. I then explored the positive interactions occurring between some sponges species and the presence of canopy-forming algae (Chapter 3). Results from this chapter suggest the canopy of Ecklonia radiata facilitates the existence of some sponge species such as Crella incrustans on vertical rocky walls. The removal of Ecklonia canopy led to a community dominated by turf algae, which corresponded with a decrease in sponge abundance and richness. My results suggest that the Ecklonia canopy facilitates the presence of some sponge species and allows their coexistence with turf algae underneath the canopy and also by altering immediate physical factors that may be detrimental for some sponge species. To further explore the existence of sponges and understory algae, I used an experimental approach (Chapter 4) to investigate the effect of the brown alga Zonaria turneriana on Leucetta sp. and also mechanisms involved in the interactions. However results from this chapter provided no evidence to support previous hypotheses that understory algae negatively affect sponges. In the last data chapter (Chapter 5), I studied sponges inhabiting different habitats in order to test if environmental variation affects the abundance and diversity of microorganisms, hence having the potential to affect the distribution and abundance of these species The stability observed in bacterial communities among specimens occupying different habitats suggests that environmental variation occurring in those habitats does not affect the stability of the community, and hence most likely does not radically alter the metabolism of these sponges. Although environmental factors such as light and sediment may have an effect on early sponge stages, other environmental (e.g. nutrients, temperature, wave action) and biotic factors, are more likely to influence the growth, survival and distribution of sponges on temperate rocky reefs. In summary, temperate sponge assemblages are strongly influenced by interactions between a number of abiotic and biotic factors. The outcomes of the ecological interactions are controlled by environment (e.g. influence of inclination on competition between sponges and understory algae) and at the same time, biological interactions (e.g. facilitation) can moderate the influence of abiotic factors such as light, sedimentation and wave action, thus facilitating the coexistence between sponge and macroalgae underneath the Ecklonia canopy. My thesis makes a significant contribution to our knowledge of temperate subtidal ecology, in terms of the effects of biotic and abiotic factors on sponge assemblages and also improves our knowledge of temperate patterns of sponge and macroalgal interactions. Finally, my thesis highlights the importance of small-scale environmental variation in influencing the structure and diversity of sponge assemblages and also increase our understanding of temperate rocky reefs sponges, especially on the less studied sponge assemblages occurring in Ecklonia stands on vertical rocky walls.</p>

A doubling of stony coral cover on shallow forereefs at Carrie Bow Cay, Belize from 2014 to 2019

To better understand the decline of one of earth’s most biodiverse habitats, coral reefs, many survey programs employ regular photographs of the benthos. An emerging challenge is the time required to annotate the large volume of digital imagery generated by these surveys. Here, we leverage existing machine-learning tools (CoralNet) and develop new fit-to-purpose programs to process and score benthic photoquadrats using five years of data from the Smithsonian MarineGEO Network’s biodiversity monitoring program at Carrie Bow Cay, Belize. Our analysis shows that scleractinian coral cover on forereef sites (at depths of 3–10 m) along our surveyed transects increased significantly from 6 to 13% during this period. More modest changes in macroalgae, turf algae, and sponge cover were also observed. Community-wide analysis confirmed a significant shift in benthic structure, and follow-up in situ surveys of coral demographics in 2019 revealed that the emerging coral communities are dominated by fast-recruiting and growing coral species belonging to the genera Agaricia and Porites. While the positive trajectory reported here is promising, Belizean reefs face persistent challenges related to overfishing and climate change. Open-source computational toolkits offer promise for increasing the efficiency of reef monitoring, and therefore our ability to assess the future of coral reefs in the face of rapid environmental change.

Local versus site-level effects of algae on coral microbial communities

Microbes influence ecological processes, including the dynamics and health of macro-organisms and their interactions with other species. In coral reefs, microbes mediate negative effects of algae on corals when corals are in contact with algae. However, it is unknown whether these effects extend to larger spatial scales, such as at sites with high algal densities. We investigated how local algal contact and site-level macroalgal cover influenced coral microbial communities in a field study at two islands in French Polynesia, Mo'orea and Mangareva. At 5 sites at each island, we sampled prokaryotic microbial communities (microbiomes) associated with corals, macroalgae, turf algae and water, with coral samples taken from individuals that were isolated from or in contact with turf or macroalgae. Algal contact and macroalgal cover had antagonistic effects on coral microbiome alpha and beta diversity. Additionally, coral microbiomes shifted and became more similar to macroalgal microbiomes at sites with high macroalgal cover and with algal contact, although the microbial taxa that changed varied by island. Our results indicate that coral microbiomes can be affected by algae outside of the coral's immediate vicinity, and local- and site-level effects of algae can obscure each other's effects when both scales are not considered.

Coral reefs in the Gilbert Islands of Kiribati: Resistance, resilience, and recovery after more than a decade of multiple stressors

Coral reefs are increasingly affected by a combination of acute and chronic disturbances from climate change and local stressors. The coral reefs of the Republic of Kiribati’s Gilbert Islands are exposed to frequent heat stress caused by central-Pacific type El Niño events, and may provide a glimpse into the future of coral reefs in other parts of the world, where the frequency of heat stress events will likely increase due to climate change. Reefs in the Gilbert Islands experienced a series of acute disturbances over the past fifteen years, including mass coral bleaching in 2004–2005 and 2009–2010, and an outbreak of the corallivorous sea star Acanthaster cf solaris, or Crown-of-Thorns (CoTs), in 2014. The local chronic pressures including nutrient loading, sedimentation and fishing vary within the island chain, with highest pressures on the reefs in urbanized South Tarawa Atoll. In this study, we examine how recovery from acute disturbances differs across a gradient of human influence in neighboring Tarawa and Abaiang Atolls from 2012 through 2018. Benthic cover and size frequency data suggests that local coral communities have adjusted to the heat stress via shifts in the community composition to more temperature-tolerant taxa and individuals. In densely populated South Tarawa, we document a phase shift to the weedy and less bleaching-sensitive coral Porites rus, which accounted for 81% of all coral cover by 2018. By contrast, in less populated Abaiang, coral communities remained comparatively more diverse (with higher percentages of Pocillopora and the octocoral Heliopora) after the disturbances, but reefs had lower overall hard coral cover (18%) and were dominated by turf algae (41%). The CoTs outbreak caused a decline in the cover and mean size of massive Porites, the only taxa that was a ‘winner’ of the coral bleaching events in Abaiang. Although there are signs of recovery, the long-term trajectory of the benthic communities in Abaiang is not yet clear. We suggest three scenarios: they may remain in their current state (dominated by turf algae), undergo a phase shift to dominance by the macroalgae Halimeda, or recover to dominance by thermally tolerant hard coral genera. These findings provide a rare glimpse at the future of coral reefs around the world and the ways they may be affected by climate change, which may allow scientists to better predict how other reefs will respond to increasing heat stress events across gradients of local human disturbance.

Critical Review and Conceptual and Quantitative Models for the Transfer and Depuration of Ciguatoxins in Fishes

We review and develop conceptual models for the bio-transfer of ciguatoxins in food chains for Platypus Bay and the Great Barrier Reef on the east coast of Australia. Platypus Bay is unique in repeatedly producing ciguateric fishes in Australia, with ciguatoxins produced by benthic dinoflagellates (Gambierdiscus spp.) growing epiphytically on free-living, benthic macroalgae. The Gambierdiscus are consumed by invertebrates living within the macroalgae, which are preyed upon by small carnivorous fishes, which are then preyed upon by Spanish mackerel (Scomberomorus commerson). We hypothesise that Gambierdiscus and/or Fukuyoa species growing on turf algae are the main source of ciguatoxins entering marine food chains to cause ciguatera on the Great Barrier Reef. The abundance of surgeonfish that feed on turf algae may act as a feedback mechanism controlling the flow of ciguatoxins through this marine food chain. If this hypothesis is broadly applicable, then a reduction in herbivory from overharvesting of herbivores could lead to increases in ciguatera by concentrating ciguatoxins through the remaining, smaller population of herbivores. Modelling the dilution of ciguatoxins by somatic growth in Spanish mackerel and coral trout (Plectropomus leopardus) revealed that growth could not significantly reduce the toxicity of fish flesh, except in young fast-growing fishes or legal-sized fishes contaminated with low levels of ciguatoxins. If Spanish mackerel along the east coast of Australia can depurate ciguatoxins, it is most likely with a half-life of ≤1-year. Our review and conceptual models can aid management and research of ciguatera in Australia, and globally.

Kepadatan Bulu Babi (Diadema setosum) Pada Ekosistem Terumbu Karang Di Karang Kering Perairan Bedukang Kabupaten Bangka

Perairan Bedukang memiliki tutupan ekosistem Terumbu Karang kering Bedukang 35,9%. Terumbu karang berfungsi sebagai tempat memijah, mencari makan, daerah asuhan bagi biota laut, dan sebagai sumber plasma nutfah. Kehadiran Diadema setosum pada ekosistem terumbu karang dapat mempengaruhi keseimbangan ekologi di ekosistem terumbu karang suatu perairan. Penelitian ini bertujuan untuk memberikan data kepadatan Diadema setosum pada ekosistem terumbu karang dan memberikan informasi keterkaitan antara kepadatan Diadema setosum dengan karakteristik habitat pada ekosistem terumbu karang di perairan Bedukang Kecamatan Riau Silip Kabupaten Bangka. Penelitian dilakukan pada bulan Maret 2019 di Karang Kering Perairan Bedukang. Metode Belt Transect digunakan untuk pengambilan data Diadema setosum dan Line Intercept Transect untuk pengambilan data terumbu karang. Hasil penelitian didapatkan nilai kepadatan rata-rata Diadema setosum sebanyak 3.708 ind/ha. Persentase tutupan terumbu karang di perairan Bedukang dalam keadaan baik dengan nilai 59,61%. Keterkaitan antara kepadatan Diadema setosum dengan bentuk pertumbuhan karang atau lifeform karang Acropora Branching (ACB), Acropora Digitate (ACD), Coral Submassive (CS), Turf Algae (TA), dan Other (OT).

Nitrogen fixation and denitrification activity differ between coral- and algae-dominated Red Sea reefs

Coral reefs experience phase shifts from coral- to algae-dominated benthic communities, which could affect the interplay between processes introducing and removing bioavailable nitrogen. However, the magnitude of such processes, i.e., dinitrogen (N2) fixation and denitrification levels, and their responses to phase shifts remain unknown in coral reefs. We assessed both processes for the dominant species of six benthic categories (hard corals, soft corals, turf algae, coral rubble, biogenic rock, and reef sands) accounting for > 98% of the benthic cover of a central Red Sea coral reef. Rates were extrapolated to the relative benthic cover of the studied organisms in co-occurring coral- and algae-dominated areas of the same reef. In general, benthic categories with high N2 fixation exhibited low denitrification activity. Extrapolated to the respective reef area, turf algae and coral rubble accounted for > 90% of overall N2 fixation, whereas corals contributed to more than half of reef denitrification. Total N2 fixation was twice as high in algae- compared to coral-dominated areas, whereas denitrification levels were similar. We conclude that algae-dominated reefs promote new nitrogen input through enhanced N2 fixation and comparatively low denitrification. The subsequent increased nitrogen availability could support net productivity, resulting in a positive feedback loop that increases the competitive advantage of algae over corals in reefs that experienced a phase shift.

Interannual variability and decadal stability of benthic organisms on an Indonesian coral reef

The availability of colonizable substrate is an important driver of the temporal dynamics of sessile invertebrates on coral reefs. Increased dominance of algae and, in some cases, sponges has been documented on many coral reefs around the world, but how these organisms benefit from non-colonized substrate on the reef is unclear. In this study, we described the temporal dynamics of benthic organisms on an Indonesian coral reef across two time periods between 2006 and 2017 (2006–2008 and 2014–2017), and investigated the effects of colonizable substrate on benthic cover of coral reef organisms at subsequent sampling events. In contrast with other Indonesian reefs where corals have been declining, corals were dominant and stable over time at this location (mean ± SE percentage cover 42.7 ± 1.9%). Percentage cover of turf algae and sponges showed larger interannual variability than corals and crustose coralline algae (CCA) (P < 0.001), indicating that these groups are more dynamic over short temporal scales. Bare substrate was a good predictor of turf cover in the following year (mean effect 0.2, 95% CI: 0–0.4). Algal cover combined with bare space was a good predictor of CCA cover the following year generally, and of sponge cover the following year but only at one of the three sites. These results indicate that turf algae on some Indonesian reefs can rapidly occupy free space when this becomes available, and that other benthic groups are probably not limited by the availability of bare substrate, but may overgrow already fouled substrates.

6. Reef fish and other major predators

Fish, like corals, have geographical patterns across regions and across individual reefs, being structured in the latter case by wave energy and depth. The thousands of species show a variety of feeding patterns. Detritus feeders are very abundant, feeding on the detritus on the seabed, especially in the fine, filamentous algal turf on apparently bare rock. Plankton feeders are common also, and herbivorous fishes show a large abundance, perhaps a quarter of the total species present, cropping algae that otherwise would grow unchecked and smother coral. Since turf algae also contain many micro-species and detritus, most herbivores also ingest much food other than simple plant material. Carnivores range from extreme specialists, such as polyp-picking butterflyfish, to generalists. Sharks and barracuda only consume other fish and generally are at the top of their food chains. The complicated ecological structure of the food webs can be clarified by analysing nitrogen isotope ratios in their tissues. Other important coral carnivores include the crown of thorns starfish, which can remove almost all living coral on a reef when it develops into plagues. Overfishing by humans greatly disturbs the equilibrium of a reef, and this is increasingly causing reef degradation.