Coral reefs are marine ecosystems present in tropical oceans around the world that provide socio-economic benefits and contribute to the health of the biosphere (Buddemeier et al. 2004, Hughes et al. 2017). Additionally, they are one of the most complex and diverse ecosystems, with high rates of primary production (Barlow et al. 2018) and aesthetic and commercial value, especially with respect to fisheries and tourism (Rendis et al. 2016).

Within reef ecosystems, algae are one of the most important components because they play a critical role in diverse ecological processes, such as forming the base of trophic chains, contributing to the formation and construction of coral reefs, and providing a habitat for various species of ecological and commercial importance (Mumby et al. 2007, Díaz-Pulido et al. 2009). One of the main consequences of coral reef degradation is the coral-algal phase shift, which refers to a change from reefs dominated by corals to reefs dominated by algae (Hughes et al. 2017). This change has a negative effect on the reef ecosystem because it includes a loss of biodiversity, evidenced by the herbivores that live in and feed on the ecosystem (Cheal et al. 2010, López-Jiménez et al. 2020), eutrophication through increased algal growth (Lapointe 1997), the mortality of coral caused by the release of macroalgae through competition for space with corals (Díaz-Pulido et al. 2009), and the rapid degradation in response to numerous anthropogenic drivers (Hughes et al. 2017). A major factor in reef degradation is sedimentation due to coastal development, dredging and other activities, which produce suspended sediments (Kroon et al. 2012). Moreover, sedimentation poses a threat to the persistence of coral reefs because it reduces recovery after disturbance events (Weber et al. 2012, Jones et al. 2016). Sediment can be released into the water column by terrestrial runoff (Fabricius 2005, Kroon et al. 2012), natural resuspension events (Orpin and Ridd 2012) and anthropogenic activities present in the coastal zone (Jones et al. 2016, Hughes et al. 2017). Sediments can either be deposited or remain in the water column, depending on their concentration, grain size, density and buoyancy, as well as the hydrodynamics of the water column (Smith and Friedrichs 2011). This is why excessive sedimentation can adversely affect the structure and function of corals and macroalgae, altering biological processes such as reproduction, metabolism, recruitment, growth, coverage and density (Babcock and Smith 2002, Florez-Leiva et al. 2010). Coral function, coralline alga recruitment and reef calcification rates are also negatively affected (Fabricius and De’Ath 2001), leading to a long-term loss of the coralline alga symbiont community and impairing the future recolonization potential of corals on disturbed reefs (Ricardo et al. 2017). These downstream effects facilitate the growth of fast-growing macroalgae such as turf algae (Kendrick 1991, Díaz-Pulido and McCook 2004).

In the present study, we investigated experimentally the effects of terrigenous sediments on different morphofunctional groups of macroalgae (filamentous turf, fleshy algae and coralline algae) to determine how these effects impact the coral reef’s ecosystem during the dry season in Capurganá Bay in the Colombian Caribbean.

A total of 66 taxa were found, including two infraspecific taxa and coral reef-forming alga species (Table 1). Of these, 33 taxa corresponded to FTA, 21 to FA and 12 to CA. The impact of sedimentation on the algae did not vary in the experiment for either additions or natural conditions (P=0.054, potentially due to a type II error) (Table 2). However, the LSD test showed differences in some treatments (addition and control 2, difference=-0.758). These analyses showed a trend throughout the experiment, in which, were observed higher values of algae recruitment and growth (mainly FTA) in the treatments with the highest levels of sedimentation and higher AC coverage on the natural conditions (control 2). With the addition of sediment, a higher affinity of the FTA was found, while the FA and CA had a greater presence in control 2.

In this experiment, it was observed that the increase in sedimentation at the levels explored had a very slight effect on the structure of the algal community in Capurganá Bay. The three functional groups of algae explored responded differently to the effects of the sediment addition, as this can change the physicochemical characteristics of the benthic zone and promote the presence and development of some algal species, primarily FTA (Birrell et al. 2005, Eriksson and Johansson 2005, Balata et al. 2007).

The addition of sediments increased the presence of FTA but had a negative effect on the presence of FA and CA. Some species, however, appeared mainly in one treatment or another, but with very low abundances (Table 1). Although no statistically significant differences were found, the observations show a trend towards a greater presence of FTA throughout the experiment, potentially due to a type II error as a consequence of the low number of replicates. It has been found that some macroalgae can take advantage of sediment-rich habitats, including thicker stalk growth, apical growth, vegetative propagation, regeneration of basic structures, temporal reproduction cycles, and growth due to fluctuations in sediment cover (Airoldi and Cinelli 1997, García and Díaz-Pulido 2006), while others can be negatively affected by these habitats (Kendrick 1991, Fabricius and De’Ath 2001). The greater presence of FTA may be due to their ability to retain or trap more sediment particles from river or continental discharges than other morphofunctional groups (Birrell et al. 2005, Quan-Young and Espinoza-Avalos 2006, Tebbett et al. 2018). This why their dominance in areas with high sedimentation rates has mainly been attributed to the fact that they act as natural sediment traps, accumulating large amounts of fine sediments (Acosta-González et al. 2013, Cetz-Navarro 2015). The turfs accumulate organic matter rich in phosphorus and other nutrients necessary for optimal growth, and this favours the proliferation of these communities over the rest of the functional groups (Purcell and Bellwood 2001, Eriksson and Johansson 2005, Quan-Young and Espinoza-Avalos 2006). They may remain in conditions of high sedimentation-unlike some corals and other algal groups-because they have developed the ability to take up and store sediment (D’Antonio 1986, Babcock and Smith 2002, Cetz-Navarro et al. 2015). On the other hand, the reduction in the presence of CA due to the increase in sediments involves some limitations for their settlement and development (Díaz et al. 1996, Doropoulos and Díaz-Pulido 2013), because they are associated with environments with low sedimentation, which is decisive for promoting calcification rates and coral settlement on reefs (Kendrick 1991, Fabricius and De’Ath 2001). High sedimentation causes suffocation in CA, causing them to lose pigmentation or discolour (bleach). It also reduces the light available for the development of these algae, which can lead to the long-term loss of the CA community, limiting future recolonization of previously disturbed reefs (Ricardo et al. 2017). The increase in sedimentation is a problem that coral reefs and CA suffer from globally (Fabricius and De’Ath 2001), because it reduces the availability of space for the settlement of coral larvae and CA, which can cause the death of some coral species by suffocation, reducing the growth of coral by abrasion and decreasing the activity of zooxanthellae among other deleterious effects (Florez-Leiva et al. 2009, Bégin et al. 2016, Ricardo et al. 2017).

Capurganá Bay is subject to a high degree of anthropogenic influence, including tourism, continental discharge of rivers, overfishing, herbivore fishing and direct sewer drainage to the sea. These stressors have caused a greater growth of algae, an accumulation of pollutants in sediments and marine species, and a reduction of light. Higher levels of nutrients and a decrease in the transparency of the water indicate a higher concentration of pollutants (suspended sediments, nitrogen and phosphorous), which leads to more algae and less coral diversity (López-Jiménez et al. 2020). It should be noted that the increase in sedimentation rates is a major problem in coral reefs worldwide, causing mortality in reef species such as hard corals and greatly increasing the space available for alga colonization (Díaz-Pulido and McCook 2004, Bégin et al. 2016). However, this space can primarily be occupied by FTA, which are excellent sediment traps and much more resistant (Airoldi 2003) than the other functional groups evaluated. The prevailing view is that the effects of sedimentation on the coral reef ecosystem depend mainly on the community structure, the interaction with other biological, physical, and chemical factors and, to a large extent, the frequency of sedimentation events in the environment (Babcock and Smith 2002, Birrell et al. 2005). Nonetheless, it is important to clarify that there are algae that have natural variations and cycles that depend completely on factors specific to each species or environmental variable characteristic of each climatic season (e.g. temperature, salinity and light intensity), without necessarily affecting the health of the coral colonies (Márquez and Díaz 2005).

In the present study, relatively low sedimentation rates were used when compared with other studies, in which sediment experiments used higher sedimentation rates and the nature of the sediment was evaluated (Babcock and Smith 2002, Florez-Leiva et al. 2009). Experimental studies with algae have shown that high levels of sediment decrease the fixation of microscopic states, limit the fixation of propagules and recruitment (Balata et al. 2007, Florez-Leiva et al. 2009), and produce a reduction in the sizes of the thallus and reproductive capacity (Birrell et al. 2005). In summary, there are a great variety of responses from algal communities to the addition of different types of sediments, and the responses largely depend on the geographic, geomorphological and natural levels and types of sediments deposited in the ecosystem. Thus, these processes promote reef degradation and can modify the population dynamics and recruitment of algae in Capurganá Bay, which may have important implications for understanding the ecology of this ecosystem.

Finally, for the treatments studied we found that the responses depended to a large extent on the characteristics and nature of the deposited sediments and exhibited high taxonomic richness, which was observed in all the substrates (plates) of the experiment. Given the complex interactions between the nature of the sediment and the algae in this study, it is difficult to make generalizations about their role in algal communities by morphofunctional groups, although these provide a relevant degree of information. Hence, in this experiment, it was important to identify the species to a more specific taxonomic category. Nonetheless, at the experimental level, this study shows the importance of the nature of the sediments and their potential effects on the structure of algal communities in these reef systems. The intensity of the sedimentation effect was evidenced by the amount of sediments added in addition to the type of algae studied. The adaptation responses of the algae to the sediments in each of the experimental treatments depended on the species involved in the interaction, their intrinsic properties, type and strategy of growth, their susceptibility to herbivores and their affinity with the sediments. These are some of the properties that may explain the patterns observed in this study concerning the frequency and appearance of one species of algae or another. Due to the multiple and intricate interactions between sediments and algae in this study, it is difficult to draw conclusions about their role in algal communities. However, at an experimental level, this work shows the importance of studying and knowing the presence of sediments and the sediments rate and their possible effects on the structure of algal communities in coral ecosystems.