4.2. Influence of Seagrass Structure and Environmental
Variables on Epifaunal Abundance
Significant variability stemming from both site and season was evident
along with a seasonal pattern in epifaunal abundance (Figure 3.1).
Abundances were observed to be at its highest during summer, but
generally decreased in cooler months. Highest abundances of epifauna
were found in denser seagrass beds at Bottelary and Geelbek, while beds
with longer leaves near the lagoon mouth supported a richer suite of
species compared to short-leaved populations, albeit in lower densities.
Despite the absence of positive and significant correlations between
epifaunal abundance and seagrass metrics, all the assessed seagrass
variables were selected as important in elucidating the variation in
epifaunal populations. Among these variables, leaf width and length
emerged as the most influential contributors to the observed variation,
accounting for 29.78% and 24.39% respectively.
The abiotic environment in Langebaan Lagoon significantly influenced
variability in seagrass morphometric parameters (Lawrence 2023, in
press ). Notably, lower temperatures were associated with larger
seagrass leaves which provide a larger surface area conducive for
epiphyte colonization (Bologna & Heck, 1999; Terrados & Medina-Pons,
2011). Faunal diversity in seagrass habitats tends to increase with
larger plant sizes and higher biomass, however other factors such as the
ecological characteristics of fauna, can also play a role in shaping
community structure (Heck & Orth, 1980; Orth et al., 1984). Given that
leaves of different seagrass species differ in area per unit biomass, it
has been shown that plants with greater aboveground foliar biomass
provide more shelter, protection and available food than smaller plants
with less surface area per biomass (Gartner et al., 2013; Hansen et al.,
2010). This concept provides a plausible explanation for the observed
variations in epifaunal abundances across different sites with distinct
seagrass plant sizes and leaf areas within Langebaan Lagoon.
Several studies demonstrate similar patterns to that observed in
Langebaan. For example, in an assessment of beds with low, medium and
high densities of Zostera marina in the United Kingdom, infaunal
diversity increased with increasing seagrass density, and significant
differences in community structure between shoot density ranges were
observed (Webster et al. 1998). Similarly, seagrass biomass was a key
regulator of macrofaunal diversity, abundance, dominance and trophic
arrangement independent of hydrodynamic and sediment properties in
Apalachee Bay, Florida (Stoner 1980). Likewise, increased habitat
complexity as a result of greater seagrass biomass was a key factor in
regulating macrofauna in Zostera capensis beds in Mozambique
(Paula et al. 2001). Z. marina shoot density exhibited a
significant relationship with total faunal abundance, with varying
effect magnitudes across 19 sites nested within three locations in the
southwest United Kingdom (Smale et al., 2019). In that study, higher
seagrass density generally corresponded to greater faunal abundance,
contributing to shifts in the structure of the faunal assemblage.
In the final structural equation model, several key factors were
identified to directly impact the patterns of epifaunal distribution
within Langebaan Lagoon. Notably, seagrass density, leaf width, oxygen,
and turbidity exhibited direct effects, while other environmental
variables played a substantial indirect role (Fig. 3.4). Although there
was no direct link between temperature and epifaunal abundance, and the
indirect effect was relatively modest (a 1SD increase in temperature
correlated with a 0.09SD decrease in epifaunal abundance), temperature
displayed a direct influence on both leaf length and width (Table 3.6).
It was also found to significantly predict five out of the six evaluated
seagrass metrics (Lawrence 2023, in press ). Indirect effects have
been shown to influence community structure as significantly as direct
effects (Wootton, 2002). Consequently, the observed decline in epifaunal
abundance in the field is likely attributed to a combination of direct
physiological effects of temperature and indirect impacts on the
underlying seagrass ecosystem (MarbĂ & Duarte, 2010).
Smaller and denser morphologies of Zostera capensis experienced
higher temperatures than their large-leaved counterparts. In seagrass
beds at Bottelary and Geelbek, higher shoot and leaf densities were
observed from smaller plants with narrower leaves, resulting in greater
leaf area per unit biomass. The lower diversity of epifaunal species and
the dominance of just two species suggest a narrow ecological niche and
a high degree of adaptation to local environmental conditions at these
particular sites. In a study by Edgar and Barrett (2002) conducted in
Tasmania, it was determined that the primary factor influencing species
richness across 48 estuaries was tidal height. Unlike seasonal
variations, spatial variance played a more substantial role in
influencing species richness, which is consistent with the findings in
Langebaan Lagoon. Those authors also revealed a relationship between
species richness, faunal biomass, and factors like salinity and seagrass
biomass, particularly during low tide and shallow subtidal levels (Edgar
& Barrett, 2002). Notably, physiological tolerance to environmental
stress, particularly in response to exposure, has been identified as a
strategy to evade the adverse impacts of predation and competition. This
phenomenon is more pronounced in diverse communities within temperate
seagrass ecosystems (Barnes & Ellwood, 2012).
Seagrass habitats serve as a structural foundation for epifaunal
communities. Habitats characterized by structural complexity often
facilitate the co-existence of a higher number of species by buffering
the effects of competition and predation (Gilinsky, 1984; Menge &
Sutherland, 1976; Russ, 1980). This heightened structural complexity
within habitats can indirectly regulate species interactions,
particularly by curbing predation. This is achieved through the
provision of increased refuge options for prey species, thereby reducing
capture efficacy of predators (Hammerschlag-Peyer et al., 2013; Menge &
Sutherland, 1976). The presence of numerous refuges are linked to
heightened prey diversity, which has been shown to balance otherwise
unstable predator-prey interactions (Heck & Orth, 1980; Orth et al.,
1984).
Enhanced species diversity is pivotal to the improved functioning and
resilience of ecosystems (Duffy et al., 2003; Unsworth et al., 2015).
While the suitability of a particular species as a food source hinges
largely on its abundance, relying on rare species for sustenance is
unlikely (Balvanera et al., 2006; Duarte, 2000). Consequently, the
preservation and enhancement of biodiversity, encompassing both species
richness and abundance, assume a critical significance in bolstering
trophic structures. This, in turn, ensures the overall vitality of
ecosystems and ensures the sustained provision of ecosystem services.
Epifaunal community structure in Zostera capensis beds was
influenced directly by seagrass leaf size and density, and indirectly by
environmental variables. Temperature notably shapes seagrass metrics,
with warmer temperatures producing smaller, denser seagrass morphologies
(Lawrence 2023, in press ). Persistent temperature increases may
prompt a shift to smaller populations with distinct faunal associations.
Further warming could narrow the range of larger populations, reducing
overall faunal diversity. Increasing temperatures would also mean
greater levels of evaporation. Rising temperatures also enhance
evaporation, potentially favoring more desiccation-tolerant species.
Fluctuations in seagrass abundance could heighten the risk of the
further threatened Siphonaria compressa limpet.
Diminished epifaunal diversity in Zostera capensis beds in
Langebaan Lagoon suggests decreased productivity and energy flow through
trophic levels, especially for fish prey. As crucial sites for
recreational and industrial fisheries in the lagoon and adjacent
Saldanha Bay, the loss of seagrass habitats implies a loss of food and
shelter for juvenile fish with critical consequences for these ecosystem
services.