Macrofauna diversity of unique, fluvially-dependent soft-sediment habitats in the Uthukela Marine Protected Area, South Africa.
Loading...
Date
2021
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
The uThukela shelf is a large section of the KwaZulu-Natal (KZN) Bight, and being situated
adjacent to the largest river on South Africa’s east coast and the fluvially-dominated uThukela
Estuary, is an excellent example of a fluvially-dependent coastal ecosystem. Previous studies
found that this shelf contains structurally and functionally unique macroinvertebrate
communities that contributed to the promulgation of the uThukela Marine Protected Area
(MPA), as part of the South African MPA network of 20 new or extended systems. This study
expands on this evidence using recent samples collected prior to the protection of the uThukela
shelf to provide a good baseline database for future monitoring within the MPA. The uThukela
macrofauna distributions and the environmental parameters correlated with these patterns were
investigated through replicated sediment grabs that were collected with corresponding abiotic
parameters, along coast-perpendicular transects. Macrofauna were subsequently classified
taxonomically and their functional attributes determined. One replicate collected on the innershelf
was particularly noteworthy as the taxa composition was unique and unexpected for a mud
depocenter as it was indicative of hard substrata, suggesting a nearby low-lying reef. This
habitat anomaly should be further investigated as it may play an important role in biodiversity
and ecosystem functioning within the uThukela benthic system that mostly consists of soft
sediments. Overall, the uThukela shelf soft-sediment community represented a wide variety of
taxa but in low abundance, and was dominated by burrowing polychaetes. These ubiquitous
polychaetes exhibited diverse biological traits, and a finding of this study was that this group
alone is a potential surrogate for future studies and monitoring of the entire uThukela shelf
macroinvertebrate community. This community consisted of mostly facultative deposit feeders
that shift to suspension-feeding and rely significantly on terrestrial particulate organic matter
(POM) and mud deposited onto the shelf by the fluvially-dominated uThukela Estuary. The
adaptive behaviour of these taxa allows intermittent shifts in food acquisition when conditions
do not favour this optimal deposition (such as during reduced fluvial outflow), thereby
enhancing ecosystem resilience to natural environmental fluxes. Natural fluctuations in the
uThukela River flow results in reduced freshwater penetration onto the shelf during the dry
(winter) season, and was observed during this study by the higher-than-expected salinity
measurements further inshore and the resultant atypical salinity gradient across the uThukela
shelf. Salinity, along with dissolved oxygen, were the measured near-bottom water parameters
most correlated with macrobenthic distribution. Sediment composition also affected
distribution patterns, forming fine-grained and medium-coarse-grained assemblages on the
inner and mid-shelf, and a muddy assemblage on the outer-shelf. The muddy outer-shelf is
mostly old deposits that suggests the uThukela has functioned as a fluvially-dominated system
for a long time. Overall, sediments contained a large amount of crushed-shell and high
Foraminifera abundances, contributing to habitat complexity and increasing diversity.
Maintaining macrofauna diversity by preserving benthic habitats is vital in the functional
success of marine ecosystems; particularly so in the uThukela system that is classified as
strongly benthic-driven. This study provides baseline information contributing to future
monitoring of whether the uThukela MPA achieves the aim of protecting rare benthic habitats
associated with the connection of the coast to the deep sea and whether macrofauna diversity
and associated ecological processes are maintained. In addition, it will support future studies
within the MPA that emphasise the importance of the critical role of freshwater to the marine
system and that ensure areas important for life-history strategies of vertebrates and invertebrates
with high conservation status are conserved.
Description
Master’s Degree. University of KwaZulu-Natal, Durban.