The significance of shore height in intertidal macrobenthic seagrass ecology and conservation

Abstract

ABSTRACT1. Benthic faunal assemblages of an intertidal seagrass bed were sampled at three shore heights (LWN, MLW, LWS) at the mouth, mid-point and head of the Steenbok Channel in South Africa's premier seagrass site, the warm-temperate Knysna estuarine bay, Garden Route National Park.2. Faunal abundance, species richness, species diversity, and proportion of rare species were relatively uniform along the Channel, as were faunal abundance, species diversity and proportion of rare species down the shore. Overall species richness per station, however, was significantly lower at LWN than at either MLW or LWS, although the distribution of species richness over the shore did not depart from random at one of the three sites. Overall faunal abundance and those of individual component species were dispersed patchily through the bed.3. The nature of the faunal assemblages present, however, varied significantly throughout the bed, both along the Channel at each shore-height horizon, and down the shore at each site. LWN assemblages formed a unit distinct from those at MLW and LWS. Overall, the shore-height axis accounted for 47% and the along-shore axis 28% of total assemblage variation.4. Faunal assemblages were randomly structured at each nodal intersection point of the down-shore and along-shore axes, but were significantly non-randomly assembled down the shore at each site and along the Channel at each shore-height horizon except at the LWN level.5. Components of total assemblage variance were largest at the smallest scale investigated (1 m). 6. Higher seagrass horizons are not just progressively more impoverished versions of lower ones and although macrofaunal assemblage composition is heavily influenced by shore height much of the ecological structure of faunal assemblages seems little affected by tidal horizon. These findings are discussed in relation to conservation of the vulnerable seagrass Nanozostera capensis at Knysna, a system affected by subsistence exploitation of its intertidal benthic invertebrates.

Context dependency in the effect of Ulva‐induced loss of seagrass cover on estuarine macrobenthic abundance and biodiversity

“…This study repeated, at a similar time of year (austral late summer–early autumn), the sets of samples from Knysna's Steenbok Channel taken in early 2011 (Barnes & Ellwood, ) when the entire shore concerned was a continuous Z. capensis bed. Conditions along this channel range from semi‐sheltered at its mouth to highly sheltered at its head, as it narrows, shallows and becomes more enclosed by saltmarsh.…”

“…One of the areas of the Knysna marine embayment showing greatest loss of seagrass was the shores of the Steenbok Channel (Figure ), from the central regions of which seagrass virtually disappeared except near low‐water neap‐tide level, whilst at the head and mouth large bare patches appeared in the beds. The southern shore of this channel happened to have been the region in which Barnes and Ellwood () had earlier carried out a sampling programme to examine the nature of the then seagrass macrobenthos in relation to both shore height and distance from the channel's mouth. In 2011 the seagrass faunal assemblages there proved to be those characteristic of beds throughout the main body of that estuarine system (Barnes, ) and to comprise classic members of the southern African estuarine fauna (Henninger & Froneman, ; Schlacher & Wooldridge, ), the channel shore then supporting some 90 species at an overall density of 6,000 individuals/m 2 , with polychaete annelids comprising 49% of the individuals, malacostracan crustaceans 17%, and truncatelloid microgastropods 16%.…”

Functional uniformity underlies the common spatial structure of macrofaunal assemblages in intertidal seagrass beds

“…Comparable data on the taxa comprising each benthic macrofaunal assemblage and on their relative importance were obtained from the datasets underlying recent published work in intertidal Zosterella habitats carried out at equivalent times of the year (Wlodarska‐Kowalczuk et al ., ) and with the same methodology: (1) on Scolt Head Island (within the Scolt Head National Nature Reserve) in the northwestern European North Sea at 53°N01°E (Barnes & Ellwood, ; Barnes, ), (2) at Knysna (within the Garden Route National Park) on the Indian Ocean coast of South Africa at 34°S23°E (Barnes & Ellwood, , ; Barnes, ; Barnes & Barnes, ) and (3) on North Stradbroke Island (within a Habitat Protection Zone of the Moreton Bay Marine Park) Queensland at 27°S153°E (Barnes & Barnes, ; Barnes & Hamylton, ; Barnes, ). All sites are hence enclosed within areas of high conservation status.…”

“…In all cases, the basic unit of comparison was the percentage relative presence of individual taxa in series of 0.0275 m 2 samples, with a minimum of 90 such samples or 25 000 individual animals in total from each locality, whichever was the smaller. The specific South African site used in comparisons with the North Sea and Queensland ones was the shoreline of the sheltered, marine Steenbok Channel in the lee of Leisure Isle within the outer basin of the Knysna estuarine bay (Barnes & Ellwood, ; Barnes, ), a site environmentally equivalent to those in the lee of Scolt Head and along the lee (Rainbow Channel) coast of North Stradbroke. Comparison of sites within the Knysna estuarine bay as a whole were between the significantly different local variants of the seagrass fauna there identified along its long axis by Barnes & Ellwood () and Barnes ().…”

Are seaward pneumatophore fringes transitional between mangrove and lower-shore system compartments?

“…whether the sediment was vegetated or not. It should be stressed that the present study did not seek to compare characteristic local mangrove faunal assemblages with those further downshore but was solely concerned with their interfaces: seagrass assemblages near their landward margin are known to be relatively species-poor and not representative of those across the intertidal zone in general (Barnes and Ellwood, 2011;Barnes and Barnes, 2012) and likewise those in the seaward margins of mangroves may not typify that habitat either (Morgan and Hailstone, 1986;Metcalfe and Glasby, 2008).…”