Possible Links Between Estuarine Pollution and Invertebrate Biodiversity

by Monica Yasunaga,
Marine conservation student

Marine degradation from coastal pollution is difficult to measure without the appropriate frame of reference.  The parameters that must be considered include the physical, chemical, and biological interactions that are taking part in an area. To understand the extent to which human-induced development and activities affect marine ecosystems, biologists can look to the bottom of the food chain for the meat of the story.  Benthic invertebrates, namely those organisms inhabiting the seafloors, are vital to the rest of the food web.  Unlike the popular macro invertebrates of the sea (i.e. octopi, squids, and sea jellies) benthic invertebrates are significantly smaller in size– from microscopic to just a few centimeters long (Figure 1 below).  On the sea floor they inhabit the surface of rocks, vegetation, coral, and the tiny spaces between sedimentary deposits.  These animals support entire food webs, provide ecological services by overturning sediment via burrowing, and are an integral part of aquatic nutrient exchange (Gray et al. 1990).  Changes in benthic invertebrate biodiversity over time may be linked to marine pollution trends in coastal ecosystems.

Typical marine benthic invertebrates (polychaete worms, snails, isopods and copepods—smaller crustaceans). Microphotograph taken by G. Carter (2000). Photo acquired from Wikimedia Commons.

Benthic Invertebrates as Conservation Measures
Depending on the environment type, it is also important to consider how the location might be affected by coastal development—anything from sewer lines, residential complexes, to heavy industrial activity, etc. Unless species richness is measured regularly over a long period, changes in biodiversity will be hard to identify in an ecosystem. Estuaries, for example, are critical locations for invertebrate biodiversity (Kaiser et al. 2011). These aquatic ecosystems are constantly in danger of runoff pollution and shoreline development (dredging, filling, etc.). They serve as nutrient cycling zones, breeding grounds for juvenile animals, and hot spots for invertebrate and vertebrate biodiversity (Wildsmith et al. 2011). Due to the importance of estuaries, there is a need to investigate the association between pollution and species richness in these habitats. Measuring invertebrate biodiversity (i.e. an applied ratio of the number of species per location) is one approach that has been used to understand the general state of coastal ecosystems (Kaiser et al. 2011).

Associations Between Marine Pollutionand Benthic Invertebrates in an Estuary

In a 2011 volume of Marine Pollution Bulletin, Wildsmith and colleagues published a study on the association between changing water quality and the benthic invertebrate biodiversity in the Swan-Canning Estuary of Australia. Data was acquired from looking at the relative species abundance of common benthic invertebrates (crustaceans, mollusks, some benthic worms, etc.) during 1986 and 2003 (Wildsmith et al. 2011). Water samples data were also obtained from the same sites for evidence of more contaminated water in 2003 than in 1986. The Swan-Canning Estuary is one of the largest estuaries in western Australia, and home many species of benthic invertebrates. The hypothesis of the study stated that the changes in estuarine water quality coincided with decreased benthic species richness upon comparing 1986 and 2003 data (Wildsmith et al. 2011).
Methodology and Results of the Study
The 4 sites sampled for benthic invertebrate species and water quality characteristics in the estuary were Dalkeith, Applecross, Matilda Bay and Deepwater Bay. The samples were taken during the mid-season periods between the 2003 winter season (Wildsmith et al. 2011). Wildsmith and colleagues compared these measurements to the 1986 data from the same 4 sites. The 1986 data were available from a previous study done by Rose and colleagues (1994) using the same four sites in the estuary (Wildsmith et al. 2011).
The invertebrates were collected with sediment cores, which were sampling devices that dug into the bottom of the estuary at each site. The contents of the cores were sorted for invertebrates using a mesh. Species richness was recorded as the number of benthic species identified in each sample from the 4 sites.
To quantify the water conditions each site was measured for dissolved oxygen, salinity, and average temperature. The measures of water temperature, salinity and dissolved oxygen at all 4 sites of Swan-Canning Estuary were taken using a Yellow Springs International Multi-parameter Hand Held Meter Model 556 (Wildsmith et al. 2011). The 3 variables, though recorded separately, were collectively regarded as general indicators of water pollution in the sample sites. Mean dissolved oxygen concentrations were lower in 2003 than in 1986, but both salinity and mean temperatures were significantly higher in 2003 than in 1986 (Wildsmith et al. 2011). Wildsmith and colleagues (2011) noted that the estuary’s sediment contained increasing amounts of copper, lead, cadmium, and zinc due to heavy coastal development (addition of yacht clubs, waterfront properties, etc.) in recent decades. This observation was not explicitly added into the experimental design, but was discussed as another possible variable for environmental pollution in the estuary.
This study tested the hypothesis that species richness changed according to estuarine water pollution. It was found that the mean seasonal number of benthic invertebrate species was very different in 2003 than it was in 1986 in the same 4 sites. Crustacean and mollusk species richness were greatly reduced in 2003 compared to data from 1986. Interestingly enough, the benthic worm (polychaetes) samples of the estuary actually increased in species diversity when compared to 1986 (Wildsmith et al. 2011). The differences in water quality parameters suggest that the waters of the Swan-Canning Estuary have become more eutrophic and generally more contaminated by human activity by 2003 (Wildsmith et al. 2011).
The study made some generalized conclusions that reveal flaws in the experimental design. First, the measures for water pollution/contamination lacked specification. The main parameters for water quality in Swan-Canning Estuary were restricted to dissolved oxygen, salinity, and temperature (and not nutrient concentration levels). While these characteristics were certainly relevant to the overall analysis and discussion of the water quality change, there was not enough evidence to support the original hypothesis. The results showed that while crustaceans and mollusks decreased in species richness by 2003, the polychaetes increased. The authors suggested that polychaetes thrived from eutrophication, but there is lack of certainty from other confounding variables that might have contributed to the results. Water conditions may contribute to decreased/increased species richness, but other aspects can too. Fishing, shoreline development, and predator-prey interactions also have significant effects on the ecosystem makeup. Moreover, nearly 20 years made up the time gap between the sets of compared measurements. It is impossible to explain the changes in invertebrate biodiversity solely on the basis of water quality measures after the amount of time passed. Sources of water quality change like waste disposal and seasonal fluctuation might not be reflected by the basic measures of dissolved oxygen, salinity, and temperature. The results from Swan-Canning Estuary remain inconclusive as long as other pollution variables are excluded.
Suggestions for Continued Study and Conservation Approach
Australia’s Swan-Canning Estuary and Others
Previous studies have also tested similar hypotheses regarding benthic populations in estuarine environments. One in particular analyzed benthic crustacean growth trends, instead of species richness, under changing water quality measures. Dr. Martin Attrill and colleagues of the University of Plymouth’s Benthic Ecology Research Group published a study in 1999 that looked at statistical models of benthic crustacean populations over a 12-year period in the Thames Estuary of the United Kingdom. This study concluded that marine crustacean growth rates decreased in correlation with higher carbon dioxide concentrations in seawater over the time period (Attrill et al. 1999).
The environmental shifts analyzed in the Swan-Canning Estuary study raises prospects for future research endeavors. Had the study collected data at regular intervals at the 4 sites (i.e. several times a year) and considered other water quality variables, there would have been more room for statistical interpretation. It is also important to note that benthic invertebrate species differ considerably in tolerance levels to water quality changes. For example, the study might have looked at different tolerance levels to eutrophication in certain benthic invertebrates. Increased nutrient content in eutrophication events may allow some species to flourish over others in the estuary, depending on species-specific resilience. Waste disposal incidents near the coast and surface run-off from industrial/agricultural facilities also need to be incorporated into the experimental design. Establishing a stronger link between water pollution and the loss of biodiversity might encourage new regulation for the protection of our coastal ecosystems.

REFERENCES

Attrill MJ, Power, M, Thomas MR (1999) Modelling estuarine Crustacea population fluctuations in response to physico-chemical trends.  Mar Ecol Prog Ser 178, 89-99

Gray JS, Clarke KR, Warwick RM, Hobbs G (1990) Detection of initial effects of pollution on marine benthos: an example from the Ekofisk and Eldfisk oilfields, North Sea. Mar Ecol Prog Ser 66, 285-299

Kaiser MJ, Burrows MT, Hughes H (2011) Evolution and ecology of marine biodiversity: mechanisms and dynamics.  Mar Ecol Prog Ser 430, 98-288

Warwick RM & Somerfield PJ (2010) The structure and functioning of the benthic macrofauna of the Bristol Channel and Severn Estuary, with predicted effects of a tidal barrage.  Mar Poll Bull 61,92-99.

Wildsmith, MD, Rose, TH, Potter, IC, Warwick, RM, Clarke KR (2011) Benthic macroinvertebrates as indicators of environmental deterioration in a large microtidal estuary. Mar Poll Bull 62, 525-538

URL: http://www.sciencedirect.com/science/article/pii/S0025326X0900229X