macroalgae

In studies on coral–algal interactions, particular attention has been devoted to corals. Focusing on the macroalgal genus Lobophora (Dictyotales, Phaeophyceae), common on coral reefs and extensively studied in coral–algal interactions, this review aims to summarize what is known and highlight the conditions necessary for Lobophora blooms, the contrasting effects on corals, and the taxonomic and functional diversity of the genus. Studies show that under normal conditions, Lobophora–coral interactions are natural and pose no specific threat to corals as long as the algal cover is controlled by coral defenses and herbivory. In contrast, disturbances freeing-up space for colonization and reducing herbivory permit Lobophora in association with other seaweeds to opportunistically take over reefs and by density-dependent negative feedbacks prevent corals from recovering. Lobophora is, however, a species-rich group and only certain Lobophora species thrive in degraded reefs, and the specificities of interactions and phase shifts will vary among species, thus stressing the importance of taxonomic identification in the study of coral–algal interactions. This review accentuates the complexity of coral–algal interactions and the importance to consider not only the taxonomy of corals and seaweeds but also their life history traits, ecology, microbiome, and the environmental settings.

MSc thesis

Shifts of coral reefs towards alternative states occur due to local and global stressors. Although global stressors are expected to increase due to climate change, anthropogenic local stressors can be addressed to prevent the loss of important ecosystem services. Identifying and understanding how human activities affect the dynamics in the benthic communities in the reef ecosystems could facilitate more effective reef restoration efforts. But how do human activities affect water quality and subsequently the benthic cover? To answer this question we look towards the coral reefs of Bonaire, home to one of the most pristine reefs in the Caribbean. We combine existing data on human activities and environmental variables with new temporal water quality and benthic cover data generated along the west-coast of Bonaire. We created two sets of models: relating the benthic cover to water quality and explaining water quality with human activity. Because our data collection extended into unexplored deeper parts of the reef we have a unique opportunity to consider the effect of local stressors along a more extensive depth gradient.

We hypothesized that areas with high nutrient loads would be reflected by benthic cover with relatively high algae, sponges and benthic cyanobacterial mats. Our results showed this to be the case for sponges and turf algae, but not for benthic cyanobacterial mats (BCM) and macroalgae. The coral and crustose coralline algae (CCA) cover were expected to be related negatively to the selected water quality variables. The models predicting the coral cover give a mixed result. Both significant positive and negative effects of nutrients on the coral cover have been found, and relatively the positive effects are stronger than the negative effects. The expectation that an increase in human activity leads to a decrease in the water quality is a lot more nuanced, but it is clear that terrestrial human activity plays an important role. The influence of depth on the effects of the water quality on the benthic covers seems to be minimal at most. As the few significant differences in water quality effects found, were more likely to be the effect of under sampling than anything else. However, these results might change as more data becomes available, narrowing both the prediction and confidence intervals and thus increasing the chance of finding significant effects of water quality on the benthic cover and clearer effects of human activity. 

For full report or more information,  please contact erik.meesters@wur.nl or gulsah.dogruer@wur.nl

Abstract

Coral reefsare experiencing large scale degradation. Motivated by the need for regular data monitoring and forquantification of the state and change of benthic and pelagic organisms,the Global Coral Reef Monitoring Networkprotocolwas executed on 18 dive sites in fished and unfished areasaround the island of Saba in the Saba National Marine Park (SNMP) in the Dutch Caribbean from March to May 2019. Pictures of the benthos were taken andanalysed with the Coral Point Count Excel extension software and fish biomass was calculated through the Bayesian length-weight-relationship. Although considerablybelow the Caribbean-wide average, coral cover around the island seems to be slowlyrecoveringfrom past diseasesand hurricane events. Coral species richnesspositively correlates with reef fish density and Serranidae species richness. As in other parts of the Caribbean, macroalgae in the SNMP arerapidly spreadingand increasingly competefor space with habitat-providing gorgonians, sponges and other benthic organisms. Incontrast toexpectations, fish density and biomass continue to increase, evenin zones where fishing is allowed. This mightbe explained by the higher availability of macroalgae that serve as food for variousherbivorous fish species, which in turn are, amongst others, the prey of predatory fish and thosehigher up in the trophic cascade. However, with the exception ofthe commercially important fish family Lutjanidae all key fish species have declinedin average size in recent years. Another findingis the increase of coral diseases. The results indicate the need for further species-specific research in order to identify the factorsthat arecausing the degradation ofthe reefs in the SNMP. A better understandingofthe interactions, ecological roles and functions of benthic and fish communities is therefore essential for the protection of reefs, that are of high value to Saba. The results of this study contribute to the adaptive management of the Saba Conservation Foundation that manages the SNMP.

Keywords: GCRMN, Reef Health Index, marine protected area, fish-benthos interaction, macroalgae, herbivory, trophic cascade, fishing, coral disease, Caribbean

Abstract: Drifting and wrack seaweeds may originate from the detachment of natural populations and transport by currents until reaching the coast. When this is part of the natural renewal process of the seaweed beds, the drift is normally multispecific. Monospecific drifting biomass are, on the contrary, originating from excessive blooming of ephemeral and opportunistic species and generally are a consequence of anthropogenic impact. Drift and wrack algae were collected at four sites at Aruba for a taxonomic survey of the floating flora in the area. A total of 72 species were identified: 7 species of Cyanophyta, 38 species of Rhodophyta, 13 species of Phaeophyceae, 12 species of Chlorophyta, and two Angiosperms. Of these, forty species are new records for Aruba. With this study, the macroalgal flora of Aruba reaches 205 taxa. Dasya puertoricensis is reported for the first time outside its type locality. At Eagle Beach the drift was dominated by deep-water species. Keywords: Aruba; floating macroalgae; floristic study; new records

Unusually warm ocean temperatures surrounding Bonaire during the late summer and fall of 2010 caused 10 to 20 % of corals to bleach (Fig. 1). Bleaching persisted long enough to kill about 10 % of the corals within six months of the event (Steneck, Phillips and Jekielek Chapters 2A – C). That mortality event resulted in the first significant decline in live coral at sites monitored since 1999 (Fig. 2). Live coral declined from a consistent average of 48 % (from 1999 to 2009) to 38 % in 2011 (Steneck Chapter 1). This increase in non-coral substrate increased the area algae can colonize and the area parrotfish must keep cropped short (Mumby and Steneck 2008). For there to be no change in seaweed abundance would require herbivorous fish biomass and population densities to increase, but they have been steadily declining in recent years. This decline in parrotfish continues despite the establishment of no-take areas (called Fish Protection Areas – FPAs) and the recent law that completely bans the harvesting of parrotfish. The other major herbivore throughout the Caribbean is the black spined sea urchin, Diadema antillarum. However, since 2005 Diadema abundance has steadily declined. Damselfishes continue to increase in abundance (except in FPAs) and their aggressive territoriality reduces herbivory where they are present. These declines in herbivory resulted in a marked increase in macroalgae (Steneck Chapter 1). Although patchily distributed, algae on some of Bonaire’s reefs are approaching the Caribbean average (Kramer 2003). All research to date indicates that coral health and recruitment declines directly with increases in algal abundance (e.g., Arnold et al 2010).
On the bright side, predatory fishes are increasing in abundance in general but increasing most strongly in FPAs. Typically, responses to closed areas take 3 - 5 years to begin to manifest themselves. Predators of damselfishes have increased significantly in FPA sites and there, damselfish abundances are trending downward. These trends are the first signs of changes in the FPAs, and they are encouraging.
Overall, Bonaire’s coral reefs today are more seriously threatened with collapse than at any time since monitoring began in 1999.
 
Monitoring Results
The abundance of live coral at the monitoring sites has been remarkably constant since 1999. However, the bleaching related mortality event (Fig. 1) resulted in the first marked decline in live coral.
Seaweed abundance (“macroalgae”) increased sharply in 2011. While the greatest increase in algae occurred at the 18th Palm site where effluent could have increased nutrient levels, most of the other sites showed marked increases in algal abundance (see Steneck Chapter 1). Coralline algae, which has been shown to facilitate coral recruitment, remains at or near unprecedentedly low levels (Fig 2). Herbivory from parrotfishes and the grazing sea urchin Diadema antillarum remains at or near the lowest levels recorded since monitoring began in 1999 (Fig. 3 and see Cleaver Chapter 5). Herbivory from parrotfish is widely thought to be most important (e.g., Steneck and Mumby 2008) but territorial damselfishes can negate parrotfishes’ positive effects by attacking grazing herbivores and preventing them from effectively grazing (Arnold et al 2010). Damselfish abundances have trended upward in recent years (Fig. 3). However, there is a hint of a reversal to this trend in the FPAs (see Arnold Chapter 3). This reversal is consistent with the possibility that areas without fishing have elevated abundances of damselfish predators such as species of groupers and snappers (Randall 1965)  
Predatory fishes including snappers, groupers, barracuda, grunts and others increased in abundance at our monitored sites (Fig. 4 and see DeBey Chapter 6a). Specific predators known to eat damselfishes (see Preziosi Chapter 6b) show variable population densities with only a hint of an increase in 2011.   
Predatory fishes increased in abundance in both biomass (most striking) and population densities (Fig. 5). While biomass of predators in FPA and control sites is identical, the population density of predators is slightly greater at FPA sites
Coral recruitment remained lower than recorded in 2003 and 2005 (Fig. 6). However, the abundance of juvenile corals was higher in 2011 than was quantified in 2009