1. Introduction
Preserving and perpetuating biological diversity is grounded in the understanding that a decline in biodiversity could disrupt ecosystem functioning and thereby jeopardize the array of vital ecosystem services benefiting society (Costanza et al., 1997; Hooper et al., 2012; McCann, 2000) A diverse ecosystem inherently entails functional redundancy, where multiple species can perform specific roles. This redundancy ensures that functional ecosystems persist even if one species faces stressors like disease or over-exploitation. Species sharing similar functions might exhibit varied responses to stressors, bolstering ecosystem resilience. This, in turn, sustains ecosystem stability and enhances the probability of recovery from stress or disturbances (Elmqvist et al., 2003).
The preservation of biodiversity is indispensable for enhancing the capacity of intricate systems to navigate change and to diminish susceptibility to species loss. The higher the stability of an ecosystem, the more significant its role in supplying critical services to society, including carbon dioxide recycling, oxygen production, and the maintenance of productive fisheries (Balvanera et al., 2006; Cardinale et al., 2012; Ghilarov, 2000; Hooper et al., 2005). Examining the diversity of fauna within marine or terrestrial systems holds significance as it enables the exploration of various hypotheses related to the mechanisms underlying species distributions and the preservation of biodiversity (de Juan et al., 2013; Virta et al., 2021).
Benthic macrofauna in coastal ecosystems are primarily composed of molluscs, amphipods, decapods and polychaetes which often play a key role as the trophic link between primary producers and pelagic consumers (Heck et al., 2008). In a typical seagrass food web, the main components include seagrass plants, periphyton, algal epiphytes, detritivores, invertebrate grazers, vertebrate grazers, meso-predators, piscivorous predators and humans (Duffy, 2006). Seagrass structure supports macrofaunal communities that predominantly occur as epifauna (living on seagrass blades), and infauna (dwelling in or on sediment surfaces) (Bologna & Heck, 1999; Klumpp & Kwak, 2005; Leopardas et al., 2014). Macrofauna serve as a vital link in the food chain, connecting primary producers with intermediate and larger predators, both directly and indirectly associated with seagrass habitats (Baden et al., 2010; Heck et al., 2008).
Seagrass habitats support a remarkable diversity of fauna and algae, even though the seagrass plants themselves have relatively limited species diversity. For instance, Orth et al. (1984) compared faunal community densities between seagrass meadows and unvegetated areas from several studies and discovered that seagrass meadows consistently supported a richer assemblage of fauna compared to nearby unvegetated areas in both tropical and temperate ecosystems. These patterns have been observed in other studies primarily attributed to the structural complexity of seagrass habitats, the abundance of food found within them, and the greater stability of sediment in comparison to bare sandflats (Bos et al., 2007; Carr et al., 2016; McCloskey & Unsworth, 2015). However, there are exceptions to this general paradigm.
Disturbances by organisms like callianassids can influence faunal assemblages by favoring burrowing infauna, but when seagrasses are present, epifauna and seagrass-specific burrowers can replace them. This ecological shift can sometimes mask differences in species diversity indices between callianassid-dominated sandflats and seagrass habitats (Barnes & Barnes, 2012). However, in cases where locations lacking callianassid bioturbators were studied, specifically in the Knysna Estuary, South Africa, seagrass habitats were found to support fewer individuals compared to bare sandflats. While seagrasses did support more species, primarily due to the presence of epifauna, the numerical differences in individuals were statistically insignificant (Barnes & Barnes, 2014). Unvegetated sandflats typically harbour a distinct suite of species (Casares & Creed, 2008). Despite often having lower species diversity compared to vegetated areas, these unvegetated sandflats make a significant contribution to overall diversity in nearshore coastal environments.
The abundance of many macrofaunal species has been found to have a positive correlation with the structure of seagrass, including biomass, canopy and the root-rhizome system (Ávila et al., 2015; Edgar, 1990; S. Y. Lee et al., 2001). In Zostera capensis (Setchell) beds in Mozambique, macrofaunal patterns were attributed to the enhanced habitat complexity created by the standing crop and biomass of seagrasses (Paula et al. 2001). This pattern is also observed in smaller seagrasses with lower structural complexity, such as Halophila decipiens . In Guanabara Bay, Brazil, H. decipiens was found to support significantly greater macrofaunal diversity, richness, and density in seagrass habitats compared to bare sand (Casares and Creed 2008).
Seagrass stems and leaves provide a substrate for the attachment of epiphytes, periphyton and encrusting algae. Epiphytes include both micro- and macroalgae, while periphyton is a mucus-like layer that coats seagrass blades comprising largely of diatoms, blue green, red, and filamentous algae, as well as particulate material, bacteria and microfauna (van Montfrans et al. 1984, Klumpp et al. 1992). Algae and periphyton are highly nutritious and easily digested by grazing invertebrates which find both food and shelter in seagrass beds (Schneider and Mann 1991, Edgar 1992, McNeely et al. 2001). Furthermore, the structural substrate provided by seagrasses for photosynthesizing epiphytes significantly enhances primary productivity in seagrass ecosystems (Borowitzka et al., 2006; Smit et al., 2005). However, an excess of algal epiphytes can become problematic for seagrasses.
Coastal water eutrophication, primarily resulting from land-use practices and effluent discharges, has been associated with seagrass loss (Burkholder et al., 2007; Cardoso et al., 2004; Ruiz & Romero, 2003). While toxic effluent can be lethal to seagrasses (Govers et al., 2014; Koch & Erskine, 2001; Macinnis-Ng & Ralph, 2004; Negri et al., 2015), an excess of nutrients in the water can promote the growth of planktonic and benthic algae, which compete with seagrasses for nutrients and reduce light penetration in the water column, further inhibiting seagrass photosynthesis (Short et al., 1995). In addition, excessive epiphytic algal growth can smother seagrasses (Nelson & Lee, 2001; Walker & McComb, 1992) and disrupt the balance of dissolved carbon dioxide and oxygen (Raun & Borum, 2013). In some cases, long-lived epiphytes can weigh down seagrass leaves, leading to breakage (van Montfrans et al., 1984). Climate warming, coupled with increased coastal eutrophication, is predicted to promote algal growth, adding further stress to seagrass ecosystems.
The influence of environmental factors, especially temperature, light (irradiance), and salinity, on seagrass growth and distribution has been extensively studied (Lawrence, 2023 in review ; K. S. Lee et al., 2007). Typically, seagrass growth exhibits a noticeable seasonal pattern characterized by increased growth during the spring and summer months, followed by a decline in autumn and winter (Dunton, 1990; Huong et al., 2003; Vermaat et al., 1987). This seasonal pattern is primarily attributed to the individual or interactive impacts of temperature and light (Clausen et al., 2014; Nejrup & Pedersen, 2008; Pérez-Estrada et al., 2021).
Salinity plays a key role in influencing the distribution, biomass, and productivity of macrofauna in estuarine and lagoon ecosystems (Al-Wedaei et al., 2011; Vuorinen et al., 2015; Whitfield, 1992). This influence is evident in the way species are distributed along a salinity gradient, ranging from seawater levels at the estuary mouth, to brackish conditions in the middle reaches, and ultimately encountering freshwater sources in the upper reaches (Allanson & Baird, 1999; Barnes & Ellwood, 2012). For instance, research conducted in 28 coastal water bodies, including estuaries, shallow bays, and lagoons in Sussex, England, identified seawater-derived salinity as the primary environmental factor driving the distribution of taxa across freshwater and saline assemblages (Joyce et al., 2005). Similarly, in the Celestun coastal lagoon in Mexico, benthic community structure was found to be spatially influenced by salinity along a gradient, while sediment characteristics had a temporal impact on species diversity without affecting abundance (Pech et al., 2007). These patterns are largely attributed to the ionic composition of the water body, which, in turn, is influenced by the salt composition of the soil and minerals introduced from runoff or seepage waters (Pérez-Ruzafa et al., 2011).
Changes in salinity levels resulting from processes such as evaporation, flooding, water abstraction, pollution, and reduced water flow have significant implications for the structure and abundance of macrofaunal communities (Allanson & Baird, 1999; Van Niekerk et al., 2013). For instance, in the Swartkops estuary in South Africa, where salinity levels are typically similar to seawater, sediment characteristics played a more prominent role in influencing macrobenthos (Mclachlan & Grindley, 1974). Following a flood event in this system, a substantial decrease in salinity occurred, leading to alterations in the distribution of macrobenthic organisms. These changes persisted for at least two years following the flood (Mclachlan & Grindley, 1974).
Teske and Wooldridge (2003) observed that endemic mud and sand fauna in estuaries in the Eastern Cape, South Africa, were primarily influenced by substrate characteristics, while salinity played a role in limiting species from marine and freshwater habitats. Langebaan Lagoon shares similarities with estuaries, despite the absence of direct freshwater input. It is believed that groundwater seepage, supported by the presence of freshwater-associated plants like Phragmites australis and Typha capensis on the southeastern banks of the lagoon, contributes to its unique conditions (Allanson & Baird, 1999; Christie, 1981). The lagoon’s enclosed nature creates a relatively stable tidal environment, with water residence times exceeding 30 days observed in the inner basin (Largier et al., 1997). Salinity levels in Langebaan Lagoon vary from the mouth to the head (Lawrence, 2023in review ), and while no effect of freshwater input was detected, increased salinities at locations like Bottelary and Geelbek are likely due to evaporation resulting from diurnal heating (Lawrence, 2023in review ) as well as in the salt marsh creeks at the head of the lagoon (Day, 1959; Flemming, 1977).
In non-quantitative surveys of Zostera capensis beds, Day (1959) noted that macrofauna in Langebaan Lagoon exhibited a distribution pattern along a salinity gradient, ranging from marine species (mainly found in Saldanha Bay) to estuarine species (predominantly occurring in the lagoon). He attributed this pattern to the shelter provided by reduced wave action and its indirect impact on the substrate, rather than temperature or salinity. Day also observed that faunal community characteristics in Langebaan were similar to those in other estuaries in the Western Cape, South Africa (Day, 1959). However, specific detailed descriptions of macrofauna associated with seagrass habitats in Langebaan, both in terms of spatial and temporal dynamics, as well as the influence of environmental variables, are currently lacking. Such information is crucial for enhancing our understanding of macrofaunal diversity in this ecosystem.
The decline of seagrasses in Langebaan Lagoon has prompted an investigation into the role of algal epiphytes and the distribution of grazing epifauna. In this study, a specific group of macro-epifauna was examined using visual counts, which differs from traditional techniques used to quantify infauna. Traditional methods are more suitable for assessing a broader range of fauna associated with both seagrass and unvegetated habitats (Raz-Guzman & Grizzle, 2001). Visual counts, conducted during daylight hours, provide estimates of faunal composition that mainly represent non-cryptic, diurnal, and sessile species (Edgar et al., 2001). However, sampling during low tide can lead to an underrepresentation of potentially important ecological species (Pearman et al., 2016). Conversely, traditional coring methods may also miss cryptic components and patterns in species distribution influenced by succession and colonization, especially among sessile and motile epibenthic communities (Moura et al., 2008). As a result, both approaches categorize a subset of the overall faunal assemblage.
This study focuses on understanding the differences in epifaunal species composition among seagrass populations in vegetated areas only. To achieve this, a subset of species primarily involved in grazing on seagrasses and epiphytes was examined. Visual assessment was considered a suitable method to gauge the extent of epibenthic diversity supported by different seagrass morphologies. Visual techniques for assessing a subset of macrofauna in their natural habitat have been successfully employed in previous studies (Källén et al., 2012; Q. Lee et al., 2012; Poulos et al., 2013; Vonk et al., 2008) to address specific research questions and uncover meaningful patterns in diversity and species composition (Kuenen & Debrot, 1995; Vellend et al., 2008). Additionally, studies in other ecosystems like coral reefs and kelp forests have primarily focused on surface fauna, as assessing infauna, while enhancing overall diversity understanding, often involves significant habitat destruction and is generally avoided. Consequently, there exists a much larger body of literature on surface fauna in these ecosystems.
The objective of this study was to assess the spatial and temporal variations in macro-epifaunal community structure, including abundance, richness, and diversity, within Zostera capensis populations in Langebaan Lagoon. The primary factors that contribute to these variations were determined, including seagrass structural elements (such as biomass, leaf characteristics, density, and epiphyte biomass) and environmental factors (i.e., temperature, salinity, pH, turbidity, oxygen levels, and chlorophyll a concentration). This research provides a foundational understanding for further investigations into trophic interactions, specifically grazing, as well as the impacts of temperature on Z. capensis (Lawrence & Bolton, 2023).