Algae

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.

One mechanism giving fleshy algae a competitive advantage over corals during reef degradation is algal-induced and microbially-mediated hypoxia (typically less than 69.5 µmol oxygen L⁻¹). During hypoxic conditions oxygen availability becomes insufficient to sustain aerobic respiration in most metazoans. Algae are more tolerant of low oxygen conditions and may outcompete corals weakened by hypoxia. A key question on the ecological importance of this mechanism remains unanswered: How extensive are local hypoxic zones in highly turbulent aquatic environments, continuously flushed by currents and wave surge? To better understand the concert of biological, chemical, and physical factors that determine the abundance and distribution of oxygen in this environment, we combined 3D imagery, flow measurements, macro- and micro-organismal abundance estimates, and experimentally determined biogenic oxygen and carbon fluxes as input values for a 3D bio-physical model. The model was first developed and verified for controlled flume experiments containing coral and algal colonies in direct interaction. We then developed a three-dimensional numerical model of an existing coral reef plot off the coast of Curaçao where oxygen concentrations for comparison were collected in a small-scale grid using fiberoptic oxygen optodes. Oxygen distribution patterns given by the model were a good predictor for in situ concentrations and indicate widespread localized differences exceeding 50 µmol L^-1 over distances less than a decimeter. This suggests that small-scale hypoxic zones can persist for an extended period of time in the turbulent environment of a wave- and surge- exposed coral reef. This work highlights how the combination of three-dimensional imagery, biogenic fluxes, and fluid dynamic modeling can provide a powerful tool to illustrate and predict the distribution of analytes (e.g., oxygen or other bioactive substances) in a highly complex system.

At Van Hall Larenstein University of Applied Sciences (HVHL) in Leeuwarden, the sea urchin Diadema antillarum has been cultivated to help restore the coral reefs around Saba and St. Eustatius (Caribbean Netherlands). The first young urchins bred in Leeuwarden were released on March 24th to the Rotterdam Zoo (Diergaarde Blijdorp). The ultimate goal is to also breed this species on Saba in order to give the sea urchin populations there a helping hand. These sea urchins keep algae growth under control, giving corals more room to grow. During this project, researchers worked closely with students from the Coastal and Marine Management program.

Repopulation of sea urchins for reef conservation

Diadema antillarum sea urchins were the main grazers of Caribbean coral reefs until over 95% of sea urchins were killed by an unknown disease in 1983. Without sea urchins grazing, algae became the dominant group on the coral reef, outcompeting coral. Today, nearly 40 years after their mass death, sea urchins have still not recovered. HVHL is working with the RAAK PRO Diadema project (2019-2023) along with project partners for the restoration of this species on Saba and St. Eustatius (Caribbean Netherlands).

©Tom Wijers

Long awaited breeding method

For the past 40 years, researchers have been trying to breed Diadema in captivity, but unfortunately have only had limited success. Breeding as been found to be very difficult. Larvae of this type of sea urchin float along sea currents for the first 50 days of their life and are sensitive to water quality and nutrient availability. However, in 2020 researchers and students from HVHL in Leeuwarden managed to develop a method for stable and consistent breeding of young Diadema.

Rotterdam Zoo

It is difficult to transport these animals on a large scale to Saba or St. Eustatius, so the first group of young urchins will find a nice new home in the Rotterdam Zoo starting on March 24th. The next step will be to breed urchins on Saba so that they can be released into the wild, strengthening the populations and helping to restore the coral reef.

More Information

For more information about this project, visit the HVHL website. For a video about this project, please visit our YouTube channel. Follow the project on Facebook for updates.

Article published in BioNews 42

Coral reef ecosystems

Tropical coral reefs are among the most productive and biologically diverse ecosystems found on earth (Odum and Odum 1955; Connell 1978; Moberg and Rönnbäck 2003). Although these reefs only cover 0.1 – 0.5% of the ocean floor they provide a home to almost one third of the marine fish species and other marine biota (Mcallister 1991; Spalding and Grenfell 1997; Spalding et al. 2001). Like rainforests, their terrestrial equivalent, the three-dimensional habitat complexity underpins the biological success of coral reef systems (Connell 1978; Grigg et al. 1984; Reaka-Kudla 1997). This structural framework is primarily provided through the precipitation of vast quantities of calcium carbonate by scleractinian corals (Goreau 1959b; Goreau and Goreau 1959; Smith and Kinsey 1976). Basic growth of coral skeleton forms the fundament of the reef and facilitates complex ecosystem functioning and niche partitioning to harbour an exceptional heterogeneity of associated biota (Connell 1978; Graham and Nash 2012; Kennedy et al. 2013; Newman et al. 2015). Ancillary to the inexpressible biological value, millions of people worldwide rely in some way on the services provided by coral reefs, most notably for nourishment, but also for services associated with tourism and coastal protection (Costanza et al. 1997; Moberg and Folke 1999; Moberg and Rönnbäck 2003). By increasing frictional dissipation of wave energy, the complex physical structure created by corals protects coastal shorelines from erosion. This has allowed humans to settle and develop coastal areas throughout the tropics. Yet, coral reefs are at present ubiquitously under pressure due to a variety of stressors associated with increased anthropogenic activity on a global and local scale.

The marine environment is continuously exposed to change, but currently this change is more and more the result of human actions (Harvell et al. 1999; Derraik 2002; Orr et al. 2005; HoeghGuldberg and Bruno 2010). The stress exerted by the natural and anthropogenic induced changing global environment works in synergy with stressors that act on a finer spatial scale. Factors such as the overharvesting of fish, pollution, eutrophication, coastal development and the introduction of invasive species can locally trigger shifts in community composition and trophic hierarchy (Hughes 1994; Hughes et al. 2003; Pandolfi et al. 2003; Hughes et al. 2007; Hughes et al. 2017). By destabilising ecosystem functioning and interactions between key species, these stressors reduce reef resilience and therewith the capacity of coral reefs to cope with globally induced sea surface temperature anomalies or ocean acidification (Pandolfi et al. 2003; Bellwood et al. 2004; Hughes et al. 2017). Reefs in the wider Caribbean region seem particularly vulnerable to anthropogenic impact (Jackson et al. 2014). By large this can be ascribed to increased local pressures associated with the unprecedented human population expansion in the region. Since the 1950s, the total population in the Caribbean has more than doubled (United Nations, Department of Economic and Social Affairs, Population, Division, 2015). Natural biological and hydrological conditions are also less favourable compared to, for instance, the Indo-Pacific region (Roff and Mumby 2012). Biological diversity in the IndoPacific exceeds 10-fold the diversity found in the Caribbean (Spalding et al. 2001; Hoeksema et al. 2017), implying limited functional redundancy in the latter (Bellwood et al. 2003; Bellwood et al. 2004; Jackson et al. 2014). In addition, the quality of Caribbean surface water is significantly impacted by discharge from major South-American rivers like the Amazon and Orinoco as well as the North-American Mississippi river. The residence time of the polluted and eutrophic water from these rivers, combined with run-off and sewage water from the numerous islands is relatively long in the Caribbean Sea due to its distinct basin-like morphological and hydrological features (Roff and Mumby 2012). As a consequence of the rapid anthropogenic alteration of the marine environment we now see an ecological degradation of Caribbean coral reef habitats that has not occurred for over 200.000 years (Pandolfi and Jackson 2006).

Abstract

A paramount challenge in coral reef ecology is to estimate the abundance and composition of the communities residing in such complex ecosystems. Traditional 2D projected surface cover estimates neglect the 3D structure of reefs and reef organisms, overlook communities residing in cryptic reef habitats (e.g., overhangs, cavities), and thus may fail to represent biomass estimates needed to assess trophic ecology and reef function. Here, we surveyed the 3D surface cover, biovolume, and biomass (i.e., ash-free dry weight) of all major benthic taxa on 12 coral reef stations on the island of Curaçao (Southern Caribbean) using structure-from-motion photogrammetry, coral point counts, in situ measurements, and elemental analysis. We then compared our 3D benthic community estimates to corresponding estimates of traditional 2D projected surface cover to explore the differences in benthic community composition using different metrics. Overall, 2D cover was dominated (52 ± 2%, mean ± SE) by non-calcifying phototrophs (macroalgae, turf algae, benthic cyanobacterial mats), but their contribution to total reef biomass was minor (3.2 ± 0.6%). In contrast, coral cover (32 ± 2%) more closely resembled coral biomass (27 ± 6%). The relative contribution of erect organisms, such as gorgonians and massive sponges, to 2D cover was twofold and 11-fold lower, respectively, than their contribution to reef biomass. Cryptic surface area (3.3 ± 0.2 m2 m−2planar reef) comprised half of the total reef substrate, rendering two thirds of coralline algae and almost all encrusting sponges (99.8%) undetected in traditional assessments. Yet, encrusting sponges dominated reef biomass (35 ± 18%). Based on our quantification of exposed and cryptic reef communities using different metrics, we suggest adjustments to current monitoring approaches and highlight ramifications for evaluating the ecological contributions of different taxa to overall reef function. To this end, our metric conversions can complement other benthic assessments to generate non-invasive estimates of the biovolume, biomass, and elemental composition (i.e., standing stocks of organic carbon and nitrogen) of Caribbean coral reef communities.

The Spaanse Water is a relatively turbid, 3.19 km2 inland bay of virtually oceanic salinities and contains the largest seagrass, algal and mangrove areas of the Curaçao Underwater Park. During 1989 and 1990, a quantitative community assessment of the larger attached flora and fauna of the seagrass and algal meadows of the bay was conducted at 151 6 m2 stations using a quadrat sampling technique.

A total of 13 different assemblages were distinguished. Shallow assemblages were dominated by Thalassia testudinum and Halimeda opuntia. As depth increased and light levels decreased, Thalassia gave way to increased coverages of especially H. opuntia, H. incrassata, Cladophora sp. and Caulerpa verticillata. In areas with significant availability of hard substrate an assemblage characterised (though not dominated) by corals was found at depths of 0–2 m, while sponges were concentrated at depths of about 4 m. The richest assemblages were found in shallow areas with high light levels and where a mix of both hard and soft substrate occurred. Assemblages with the lowest species richness were typically associated with low light intensities, soupy muds or homogeneous sandy sediments of high grain size.

In 1930 Mr. P. Wagenaar Hummelinck made an excursion to Curacao, Aruba, and Bonaire with the main object of studying the land and freshwater fauna. In 1936 and 1937 he again visited these islands and, moreover, a.o. the island of Margarita off the Venezuelan coast, the Venezuelan peninsula Paraguana and the Colombian peninsula La Goajira (Wagenaar Hummelinck, 1940). In the various inland-waters also Algae and Phanerogams have been collected. The aquatic Phanerogams were described by Van Ooststroom (1939); the Charophyta will be the subject of the present paper.

As a result of these trips only two species of Chara were collected, one of which, viz. C. fibrosa , was new for the area under discussion. No representative of the other Charophyta genera was detected. Though several species are recorded from the north coast of South America (cf. Braun, 1858; Braun & Nordstedt, 1882), so little is known of the Charophyta of the Netherlands West Indian islands that it is worth publishing these few notes. Moreover, a number of ecological data were gathered, which are enumerated at the same time.

There has been a recent increase in public awareness of environmental issues as the effects of climate change have become ever more noticeable in our daily lives. As we enter a new decade, it becomes useful to review what conservation efforts have worked so far, and take inventory of what efforts will be required for the future. Starting with the constitutional referendum creating the Caribbean Netherlands (Bonaire, St. Eustatius and Saba (BES), the response to conservation challenges of all six Dutch Caribbean islands have varied. Since 2010, the BES islands have seen an overall increase in funding support and conservation actions, and therefore presumably also saw greater improvements when compared to Aruba, Curaçao and Sint Maarten, though clearly not enough (Sanders et al, 2019).

The goal of this Transboundary Species special edition of BioNews is to provide an update on the latest published research results and highlight the need for transboundary protection. These species know no boundaries, and thus move between the Dutch Caribbean islands and beyond. Their protection will require broadscale conservation efforts which cover the entire Caribbean, including the six Dutch Caribbean islands. Collaboration between all six islands is of the utmost importance. This is one of the Dutch Caribbean Nature Alliance’s (DCNA) main goals: working together and sharing skills, knowledge and resources to maintain a solid network and support nature conservation in the entire Dutch Caribbean.

Macroalgae on Bonaire
Macroalgae are large algae, also called seaweeds, that are typically divided in three major groups: red macroalgae (Rhodophyta), brown macroalgae (Phaeophyceae), and green macroalgae (Chlorophyta). Over 250 seaweed species are known from Bonaire. They vary tremendously in shape and color and are found in a range of habitats. They flourish in shallow and deep areas on coral reefs all around the island, in seagrass beds, mangrove forests and in the intertidal.

Macroalgae – important organisms
Macroalgae are mostly notorious as aggressive competitors for space that can overgrow reef corals. However, macroalgae play an important part in all marine ecosystems: they provide food for herbivores, and they stabilize the structure of reefs. Algae are also remarkable in that they are responsible for the high productivity that characterizes coral reefs and seagrass beds.

Identifying macroalgae
These identification cards provide an overview of almost 60 red, brown and green seaweed species that are frequently encountered on Bonaire, to help you explore the macroalgal biodiversity in the marine parks.

On the shore of the Boca Jewfish area of the Lac, Bonaire, N.A., blue-green algae perform a sediment-stabilizing and binding function resulting in a wide variety of cryptalgal structures. The morphology and zonadon of these structures is related to variation in desiccation, sediment influx, water agitation and algal "species." The zunation of intertidal structures consists of stromatolites and oncolites, lithifiod nodules, smooth mat and tufted mat. A cryptalgal crust pavement is found in protected supratidal- areas. In the middle intertidal zone, cryptalgal nodules are lithified during intertidal exposure by pervasive pore-reducing, micritic, high-Mg calcite cement, which is pendent in its distribution around sediment grains. Calcium carbonate cement also occurs as rinds on algal filaments. Precipitated calcium carbonate is found in minor amounts on filaments and mucus within tufted and smooth mats. The preservation potential of the nodules is enhanced by rapid and early cementation. The other structures, not lithified by significant amounts of early cement, have lower preservation potentials. The normal marine salinity of the Lac indicates that growth of cryptalgal structures in fresh, brackish, or hypersaline waters is not essential for their early cementation and lithification.