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).