State of the Bay Report 2010-Final - Anchor Environmental

8 BENTHIC MACROFAUNA
8.1 Background
It is important to monitor biological criteria in addition to physio-chemical and
ecotoxicological variables as biological indicators provide a direct measure on the state of the
ecosystem. Benthic macrofauna are the biotic component most frequently monitored to detect
changes in the health of the marine environment. This is largely because these species are short
lived and therefore their community composition responds relatively rapidly to changes in
environment quality over time (Warwick 1993). Given that they are also relatively non-mobile (as
compared with fish and birds) they tend to be directly affected by pollution and they are easy to
sample quantitatively (Warwick 1993). Furthermore they are well-studied scientifically, compared
with other sediment-dwelling components (e.g. meiofauna and microfauna) and taxonomic keys are
available for most groups. In addition, community response to a number of anthropogenic
influences has been well documented.
Benthic macrofauna community responses to anthropogenic impacts include changes in the
growth rates of individual organisms, changes in reproductive output, and mortality, all of which can
lead to changes in the relative abundance of individual species and hence community composition,
reduction in the number of species present, and even complete disappearance of benthic organisms
under severe circumstances (Warwick 1993). Common anthropogenic impacts of benthic
macrofauna communities include physical disturbance (e.g. dredging), increased organic inputs
(eutrophication) which can lead to hypoxia or even anoxia in sediment, elevated levels of trace
metals, hydrocarbons and/or other anthopogenically produced compounds (e.g. pesticides) in
sediments.
The main aim of monitoring the health of an area is to detect the effects of stress, as well as
to monitor recovery after an environmental perturbation. There are numerous indices that can be
used to examine changes in community structure and hence ecosystem health, which are based on
benthic invertebrate fauna information. These indices include those based on community
composition, diversity and species abundance and/or biomass. Given the complexity inherent in
environmental assessment it is recommended that several indices be used (Salas et al. 2006). The
community composition, diversity, and species abundance and biomass of soft bottom benthic
macrofauna samples, collected in Saldanha Bay and Langebaan Lagoon in 2010, are considered in
this chapter.
8.2 Historic data on benthic macrofauna communities in Saldanha Bay
The oldest records of benthic macrofauna species occurring in Saldanha Bay date back to the
1940’s, prior to the construction of the iron-ore jetty and Marcus Island causeway. Available data
from this time is mostly anecdotal (i.e. non-quantitative), and is not comparable with subsequent
studies, and as such cannot be used for establishing conditions in the environment prior to any of
the major developments that occurred in the Bay. Moldan (1978) conducted a study in 1975 where
the effects of dredging in Saldanha Bay on the benthic macrofauna were evaluated. Unfortunately,
this study only provided benthic macrofauna data after the majority of Saldanha Bay (Small Bay and
Big Bay) had been dredged. A similar study conducted in 1975 (Christie and Moldan 1977),
examined the benthic macrofauna in Langebaan Lagoon, using a diver-operated suction sampler,
and the results thereof provide a useful description of baseline conditions present in the Lagoon
from this time.
State of the Bay 2010: Saldanha Bay and Langebaan Lagoon 152