4.1. Richness, Abundance and Diversity of Seagrass
Epifauna
Despite fairly stable salinities in the lagoon, species richness and
diversity displayed a similar pattern of decline from the mouth as
observed in typical estuaries where variation in faunal communities are
largely influenced by a salinity gradient (Allanson & Baird, 1999;
Barnes, 2010; Heymans & Baird, 1995). However, unlike typical
estuaries, salinity levels in the lagoon increased with distance from
the mouth likely due to evaporation and the lack of distinct inflow of
freshwater. Variation in epifaunal community structure is not
necessarily attributed directly to salinity but could be an indirect
effect since salinity is often correlated with other environmental
variables (Yamada et al., 2007). Factors such as current velocity, wave
intensity, competition, and predation, which were not assessed in this
study, could account for the unexplained variation in community
structure.
In a similar enclosed bay with no freshwater input and a relatively high
salinity range (22 - 35) in Arcachon, France, macrofauna in beds of
another dwarf-eelgrass, Zostera noltei showed similar patterns to
that observed for Langebaan Lagoon (Blanchet et al., 2004). Species
richness was lowest in assemblages furthest from the mouth and at higher
tidal levels while abundances were highest at these places. In that
study, distance from oceanic waters (km) was used as an environmental
factor and was the main attributor of differences between assemblage
groups along with tidal range (Blanchet et al., 2004). Similarly, Barnes
and Ellwood (2012) found richness and diversity to be highest inZ. capensis beds below mean low water levels in the
marine-dominated section of the Knysna estuary, South Africa. In that
large estuarine system species richness did not display the classic
decline with distance from the mouth, but instead had a stable fauna
separated by abrupt changes in areas where salinity levels fell below 30
(Barnes & Ellwood, 2012). In Arcachon, species occurring in areas of
relatively low salinities were not distinct but rather a subset of those
present at the mouth, which was not the case at Langebaan. Several
species were found only at sites closer to the mouth and not at
Bottelary and Geelbek. In contrast, a stable faunal composition was
observed at Knysna despite the wide range in salinity
(<5–35). While the observed variation in Knysna was
supposedly due to changes in relative abundance of dominant species, or
the lack of marine species in upstream localities and upper boundaries
of seagrass beds (Barnes & Ellwood, 2012), species variation in
Langebaan was more likely a result of exposure and tolerance to
desiccation (this study), availability of seagrass habitat (Angel et
al., 2006) as well as the ability to survive under periods of exposure
to sun and wind during low tide (Day, 1959).
In Langebaan Lagoon, epifaunal community structure was dominated by
grazers i.e., Siphonaria compressa , Fissurella mutabilisand Assiminea globulus . A. globulus was the main species
recorded at Bottelary and Geelbek and occurred in lower abundances in
high shore beds at Oesterval and Klein Oesterval, although high
abundances were previously observed at Oesterval in both seagrass and
saltmarsh vegetation in the high shore (Day, 1959). Regarded as a broad
niche generalist and capable of adapting to a range of environmental
conditions, this mud snail feeds mainly on bacteria and diatoms found on
sediment surfaces and occasionally on epiphytic periphyton and is
strongly averse to prolonged periods of submergence (Angel et al.,
2006). Gastropods have been documented to dominate many estuarine
systems (Allanson & Baird, 1999) and Barnes (2013) recorded a dominance
of gastropods including 125 times greater average densities ofAssiminea in upper-estuarine and enclosed sites compared to
elsewhere in the Knysna estuary. In Arcachon, seagrass beds at high
shore sites and sites further from the mouth were dominated by a single
gastropod, Peringia ulvae (Blanchet et al., 2004). The
prevalence of gastropods in high shore areas is likely attributed to
their ability to resist desiccation supported by the upward expansion of
habitat provided by seagrass and saltmarsh vegetation.
High shore sites in Langebaan were also dominated by the pulmonate
limpet Siphonaria compressa which has been identified as South
Africa’s most threatened marine invertebrate. Classified as Critically
Endangered by the IUCN (Kilburn, 1996), this limpet has an extremely
narrow habitat range and grows only in Zostera capensis meadows
in Langebaan Lagoon and the Knysna estuary (Allanson & Herbert, 2005).
In this study, densities of 100 m-2 were recorded at
Geelbek and Bottelary and <5 m-2 at
Oesterval. None were found at Klein Oesterval and Centre Banks, however,
Angel et al (2006) found densities between 40-80 individuals per
m2 at the lower edge of seagrass beds at Klein
Oesterval over a 30 year period. Analyses of faecal pellets revealed
that S. compressa primarily feeds on bacteria, diatoms and
filamentous algae found on seagrass leaves (Allanson & Herbert, 2005).
High abundances of the two dominant grazers Assiminea globulusand Siphonaria compressa at the same sites suggest a lack of
competition and the exploitation of different niches. This hypothesis
was explored by Angel et al. (2006) who assessed the interaction betweenA. globulus, S. compressa and Zostera capensis cover in
relation to low tide exposure at Langebaan. They found that S.
compressa thrived on the lower edge of seagrass beds subjected to
shorter periods of exposure and suggested that the virtual absence of
the limpet from the upper zone was largely the result of avoiding
physiological stress from desiccation - a consequence of its small size
and thin shell. Experimental transplants of seagrass into sandbanks saw
a proliferation of S. compressa and concluded that confinement of
the limpet to beds in the high shore was suboptimal and essentially due
to the restriction of the seagrass beds themselves to expand further
into the low shore due to sediment disturbance by the burrowing sand
prawn, Kraussillichirus kraussi (Angel et al., 2006). The
conclusions of that experiment do not explain the absence of S.
compressa from low shore beds at Oesterval and Centre Banks in this
study. Here, a likely reason is competition with grazing gastropods
(Orth et al., 1984) such as Fissurella mutabilis, which is less
tolerant to desiccation and occurred in high abundances at low shore
sites close to the mouth and but not at high shore sites i.e., Geelbek
and Bottelary.
In contrast, Angel et al. (2006) recorded high and low densities ofAssiminea globulus on the upper and lower edges of seagrass beds
respectively and noted a positive correlation with exposure. Once again,
this was attributed to disturbance of sediment and reduction of diatoms
due to bioturbation, however these snails also appeared to prefer
exposure rather than submergence (Angel et al., 2006). This species’
preference for more exposed parts of seagrass beds is likely related to
the availability of food in the form of sand-dwelling diatoms that are
abundant in the high shore zone (G.M. Branch, unpublished data), and
therefore occupies a different niche to that of Siphonaria
compressa . Given that S. compressa likely feeds essentially on
periphyton on seagrass leaves (Allanson & Herbert, 2005), there was no
evidence to indicate that abundance or zonation of these two species had
been influenced by competition.
Environmental conditions such as a longer tidal emergence period at
Bottelary and Geelbek correspond to a higher susceptibility of species
to desiccation. This explains the lower species richness at these sites,
and the high abundances of the desiccation resistant A. globulusand attests to the resilience of S. compressa to persist in
suboptimal conditions. The cushion star Parvulastra exigua and
keyhole limpet F. mutabilis were previously reported to have
completely declined at Klein Oesterval (Pillay et al., 2010) but was
subsequently recorded in this study. In addition, Pyura
stolonifera , Sargartia ornata and Oxystele antoni not
documented in that survey, were also found. These findings underscore
the considerable variability inherent in seagrass ecosystems. The
elevated abundances of grazers further emphasize the important trophic
role of grazing within both seagrass and intertidal ecosystems more
broadly (Asmus & Asmus, 1985; van Der Heijden et al., 2020).
Species richness and densities recorded in this study were comparable to
the survey of macrofauna within seagrass and un-vegetated sandflats at
Klein Oesterval using the traditional method of digging and sieving
(Pillay et al. 2010). That study yielded a total 27 species and while
infauna (n = 15) was not targeted in this study, a total 13 species were
documented at Klein Oesterval - nine species were recorded in both
surveys, and an additional four were recorded in this study only. These
patterns allude to the strengths and weaknesses in sampling techniques,
many of which fail to capture various components within faunal
assemblages influenced by processes such as life history stages,
succession and colonization (Moura et al., 2008) as well as temporal
restrictions related to tidal regime, season and day/night sampling.
While traditional methods are considered comprehensive in providing
estimates of biodiversity, alternative methods including the use of a
selection of indicator species in single or multimetric indices, can be
useful to assess the ecological status of benthos in marine and
estuarine environments (Borja et al., 2011; Dauvin et al., 2016)
particularly when resources are limited. Using a subset of species has
been found to provide meaningful descriptions on faunal community
structure (Kuenen & Debrot, 1995; Vellend et al., 2008). Importance
should therefore be placed on consistency of methods used as well as
regularity of surveys to provide informative results to address research
and management objectives (Magurran et al., 2010).