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1.1 Overview of Soft Sediment Communities
Soft sediments are the most common marine habitat on earth (Wilson, 1990). The habitats include sandy beaches, estuaries, mudflats and salt marshes. The communities consist of organisms which live on, or in, the bottom of a water body. There are generally four types of soft sediment communities which can be classified according to their size. They are microbenthos (<0.063 mm), meiobenthos (0.063 - 1.0 mm), macrobenthos (>1.0 mm) and megabenthos (> 10.0 mm).
This study is focused on macrofauna, also known as macrobenthos. They are invertebrates that live on or in sediment, or attached to hard substrates. The common soft-sediment communities that can be found in intertidal areas are Annelida, Crustacea and Mollusca (Munari & Mistri, 2008).
Estuarine and coastal ecosystems consist of important components of macrofauna (Borja et al., 2000). They connect primary producersand organic matter sources such as phytoplankton and detritus apart from being economically, ecological, and recreationally importantfish and crustaceans (Rönnbäck et al., 2007; Bremner, 2008).Soft sediment communities provide many ecosystem services that help to maintain good water and sediment quality (Rönnbäck et al., 2007).
Filter feeders such as bivalves remove particles from the water column, which may result in enhanced water clarity (MacIsaac, 1996). Given the importance of light in shallow water estuarine ecosystems, filter feeding may improve shallow water habitat for submerged aquatic plants and benthic microalgae.
The degradation of some pollutants is enhanced by sediment mixing (bioturbation) of the infaunal macrobenthos due to stimulation ofmicrobial processes. The enhanced coupling of key nitrogen transformations in the presence of benthic macrofauna can lead to the production of nitrogen gas, which escapes to the atmosphere, thereby reducing nitrogen loading in the ecosystem.
Macrobenthos have been used for decades asindicators of environmental statusand trends in estuaries and coastal areas because infauna are mostly sedentary organisms and they respond to local environmental impacts (Pearson & Rosenberg, 1978; Borja et al., 2000; Wildsmith et al., 2009, 2011). They cover a wide range of physiological tolerances, living positions, type of feeding and trophic interactions (Elliott et al., 2002). Macrobenthic assemblages respond relatively quickly to habitat disturbances (Borja et al., 2000). They are important components of aquaticfood webs (Rönnbäck et al., 2007)and they affect transport and cycling of nutrients and toxicants.
In addition, there are data on their patterns of variation, their responses to different forms of disturbance are known and they show similar responses at different levels of taxonomic resolution (Warwick, 1988). They form an important component of the estuarine food-web, supporting commercial and non-commercial species. They therefore represent an ideal assemblage to measure environmental change and will continue to be used to represent an important biological component of soft sediments. Understanding how different components of benthos respond to changes in properties of sediments is therefore essential in determining how much, if any, redundancy there is in this system and how much impacts on the sediments themselves are tolerated by the fauna. From this study it is clear that such experiments need replication at multiple scales and across multiple habitats before any general responses will be identified.
By knowing the importance of soft sediment communities, the health and quality of an ecosystem can be determined without using any harmful chemical indicators.
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The objectives of this project are:
- To investigate the abundance and distribution of soft sediment communities in Tanjung Bungah, Pulau Pinang.
- To relate the environmental variables with soft sediment communities distribution and abundance in Penang intertidal area.
- LITERATURE REVIEW
2.1 The Ecology of Soft Sediment Communities
Macrofauna make up a large component of the food web in estuarine ecosystems, connecting primary producers to top producers and playing an important role in system dynamics (Herman et al., 1999; Platell et al., 2006). Bottom macrobenthic communities include a great variety of organisms and generally a large number of species and they are extremely complicated (Meire et al., 2005).
In marine macrobenthic organisms, polychaetes is one of the most significant groups and may make up more than half of the organisms in soft bottom habitats. They are often the predominant macrobenthic taxon in these sediments in terms of numbers, both numerically of species and abundance (Wildsmith et al., 2009, 2011). Polychaetes could hence be good indicators of species richness and assemblage models in macrobenthic assemblages (Fauchald & Jumars, 1979). Some polychaete species were greatly opportunistic and responded quickly to environmental disturbances (Norkko et al., 2006; Wildsmith et al., 2011).
2.2 The Importance of Soft Sediment Communities
Invertebrates constitute part of marine ecosystems and play important roles to support the function and stability of the food chains and ecosystems upon which other animals rely (Snelgrove, 1998). They regulate populations of other organisms (plant and animal) through predation, parasitism and herbivory, and help maintain water quality by filtering large amounts of water during feeding (Ponder et al., 2002). Invertebrates are directly involved in ecosystem stabilization, shoreline protection, energy and nutrient transfer and provision of habitat (Ponder et al., 2002). They also help in climate stabilization and re-mineralization and play an important role in the cycling of nutrients, breakdown of plant matter and other detritus and provide habitat for other species (Ponder et al., 2002).
2.2.1 Environmental indicator
Members of the macrofauna community serve as useful biological indicators of environmental change and key elements of many marine and estuarine monitoring programs, due to their sedentary lifestyles and reduced responses to environmental changes (Tweedley et al., 2012). Therefore, the spatial distribution of macrofauna relative to environmental factors is fundamental to the understanding of estuarine ecology (Herman et al., 1999).
Polychaetes are valuable marine organisms which can tolerate contamination because they live at the interface of water-sediment (Wildsmith et al., 2009, 2011). This layer is both biologically reactive and chemically active (Rhoads & Young, 1970). Polychaetes occupy almost all marine and estuarine sediments (Fauchald, 1977) and are often the predominant constituent of the macrobenthic communities both in terms of individuals and number of species (Hutchings, 1998; Morin, 1999; Mills, 1969; Rhoads & Young, 1970; Van Hoey et al., 2004; Ward & Hutchings, 1996; Warwick, 1988).
Polychaetes carry out an important role in ecosystem processes of macrofauna assemblages such as recycling, pollutant metabolism and in the interment of organic matter (Hutchings, 1998).
2.3 Factors Affecting Soft Sediment Communities
The abundance and distribution of soft sediment communities were influenced by both biotic and abiotic factors. Biotic factors such as competition and predation (Rhoads & Young, 1970) while abiotic factors such as variation in salinity, turbidity, sediment grain size, total organic carbon, and metal contamination, affect estuarine macrofauna (Kinne, 1966; Remane & Schlieper, 1971; McLusky & Elliot, 2004).
The influence of salinity in particular is an important factor in estuarine macrofaunal diversity. Generally, studies have reported a positive correlation between biodiversity and salinity (Holland et al., 1987; Jorcin, 1999; Ysebaert & Herman, 2002; Gimenez et al., 2005). Because variation in salinity differs in accordance with tidal movements and freshwater inputs into estuaries, the distribution of macrofauna and the macrobenthic community can differ between dry and rainy seasons.
2.3.2 Sediment Grain Size
In estuaries, salinity and sediment grain size have been found to be the most important environmental variables controlling the diversity and distributional patterns of macrofauna (Absalo, 1991; Yoo & Hong, 1996; McLusky & Elliot, 2004). Hong & Yoo (1996) suggested that the particle size and disturbance of the sediment may have been the most important factors controlling the macrobenthic community.
Soft-sediment communities are unusual in the rate at which the nature of the physical environment can change (Wilson, 1990). Most sedimentary particles are smaller than the resident organisms, infauna (Wilson, 1990). The activities of the infauna can dramatically change the nature of the environment over time periods of hours or days (Wilson, 1990). For instance, burrowing infauna may increase the porosity of the sediment (Rhoads, 1974).
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2.3.3 Total organic carbon
Population dynamics of benthic suspension feeders, deposit feeders and subsurface feeders are known to respond differently according to nature of inputs, plankton and/or organic food matter (Austen et al., 1991; Beukema et al., 2002). The latter two groups are less affected since they utilize a large pool of organic matter in the sediment, which is constantly being recycled. In tropical region, Wolanski et al. (1992) hypothesized that in mangrove swamp or creek system, the circulation processes are highly complicated leading to novel sediment transport regimes that could in turn account for the chemistry and biology of the recipient water.
- Abundance and Composition of Soft Sediment Communities
The study of soft sediment communities was extensive in certain temperate countries especially Australia while other countries in the tropics are lacking in their data. Further studies have yet to be conducted in tropical coastlines due and increasing human population growth, pollution, urbanization at an alarming rate (Hatcher et al., 1989).
- Distribution of Soft Sediment Communities in the World
In East Antarctica, Stark (2000) had investigated the distribution and abundance of soft-sediment macrobenthos around Casey Station. Two locations were compared – two potentially polluted locations and two control locations in an asymmetrical design (Stark, 2000). Stark (2000) found out that the dominant assemblage were crustaceans while polychaetes’ assemblage was smaller in number, about 3-10% of individuals at the locations sampled.
Another study in Brown Bay, Antartic which was carried out by Stark et al. (2005) revealed significant correlations between the presence of contaminants and the distribution and composition of soft sediment-communities over very small spatial scales. Combinations of certain metals, for example Cadmium (Cd), Copper (Cu), Tin (Sn) and Lead (Pb) and sediment grain size were the variables that best linked the community patterns at Brown Bay. This is further supported by previous studies in Antartic where benthic assemblages probably patchy due to local environmental conditions (Stark, 2000; Stark et al. 2003).
In England, annelids were found to be the most abundant group followed by crustaceans, molluscs, echinoderms and others (bryozoans and cnidarians) with 34.5%, 20.0%, 16%, 2.5% and 27% respectively (Bolam et al., 2008). Sediment grain size significantly affects the abundance of macrofauna along the English Channel (Bolam et al., 2008).
In Australia, polychaetes were found the highest, and crustaceans were the lowest in macrofauna abundance (Morrisey et al., 1992).
In Norway, environmental variables such as productivity, temperature and sediment grain size played a vital role in determining pattern of species richness (Gray, 2002).
In Germany, the most abundant taxonomic group was gastropods, followed by oligochaetes, polychaetes and crustaceaans with 87%, 6%, 6% and 2% abundance respectively (Schückel et al., 2013). The main causal factors for the different distribution patterns of intertidal macrofauna species which results in characteristic zonation patterns were sediment grain size and food availability, expressed in chloropohyll a contents (Schückel et al., 2013).
In Italy, the highest numbers of species that were identified were for Annelida which recorded 108 species, Crustacea recorded 69 species, and Mollusca recorded 52 species (Munari & Mistri, 2008). Changes in the composition of assemblages of local species and the dominance of annelids species cannot be explained by only one factor (Lardicci et al., 1993). The factors were linked to biotic and hydrodynamic determinants, dissolved oxygen, grain size and organic content of the sediments (Munari & Mistri, 2008).
- Distribution of Soft Sediment Communities in Asia
In Qeshm Island of Iran, Nassaj et al. (2010) investigated the abundance and distribution of macrofauna in Salakh coastal region waters (Qeshm Island-Persian Gulf). Nassaj et al. (2010) found that Polychaeta (54.14%) were the most dominant group followed by the Crustacean (27.24%), Amphipods (9%), Gastropoda (8%), Bivalvia (7%), Copepoda (2%) and other groups (4%).
In Korea, Yu et al. (2012) carried out a research on the effects of environmental variables on the distribution of macrofauna in the Han River Estuary during summer and spring. The dominant species were polychaetes during July, 2006 and March, 2007 (Yu et al., 2012). The most important factor was salinity and other factors such as sediment grain size and dissolved oxygen being secondary (Yu et al., 2012).
- Distribution of Soft Sediment Communities in Malaysia
In Pulau Pinang, the high percentage of organic matter has shown a positive relation with abundance, diversity and richness of macrobenthic (Gholizadeh, Yahya, Talib, & Ahmad, 2012). High percentages of sediment with grain size ≥125 μm revealed to have an increased in macrobenthic abundance (Gholizadeh, Yahya, Talib, & Ahmad, 2012). This may aid in expounding the higher abundance of macrobenthic organisms, particularly for the deposit feeders. It has been reported that the sediment type (sand vs. mud) is one of the parameters responsible for the spatial distribution of macrobenthic families according to feeding kinds (Rhoads & Young, 1970; Hutchings, 1998; Van Hoey et al., 2004).
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