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.