Metal composition of zooplankton from the Western Indian Ocean
Van Aswegen, J.D.
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The Indian Ocean is probably the least researched ocean of the world. It is the third largest ocean and has many unique topographic, geologic, oceanographic, biological, and socio-economic features. The current system is also complex and variable on small and large scales. Uniquely, there is a component of monsoon seasonality affecting currents. In terms of zooplankton, little is known, and even less is published on metals, a major component of a class of pollutants that also occurs naturally. Due to development, trade, shipping (especially oil), an expanding human population, coupled with current and potential conflict, knowledge on the current state of pollution (or lack of) is needed to tract future impacts and development. Plankton is one of the most important components of the ecology of any ocean. Pollutants can affect plankton, change the characteristics of pollutants, and can be assimilated and/or bioconcentrated. They also play major roles in the carbon and nitrogen cycles and the microbial loop. Disturbances of these major processes may affect ecosystem functions and affect human communities depending of the ocean. Metals, although all occurring naturally in seawater, may be elevated beyond natural background levels due to pollution, posing risk to biota. Although some ecotoxicological work has been done on mammals, fish, birds, and coral, there is a great lack of knowledge regarding metal composition in zooplankton from the western Indian Ocean (WIO), the region where I conducted my research. In this region, I concentrated on the metal compositions of three currents. The South Equatorial Current (SEC) is the main current that carries warm water from the east towards the African continent. This current runs through the tropical region of the Indian Ocean where it branches into the Madagascar Current (MC) flowing south, and East African Coastal Current (EACC). Each of these currents have upwellings and eddies. The two latter currents also receive freshwater discharges (containing metals) from many rivers along the coast, while volcanic activity in the SEC might contribute metals as well. I collected 94 zooplankton samples from the three currents, and analysed them with ICP-MS for 34 metallic elements. I analysed the data statistically but also looked at geographic interpolation. Although I analysed 34 elements, I concentrated on 14 essential (V, Cr, Mn, Fe, Co, Cu, Zn, As, and Se) and five non-essential (Ni, Cd, Hg, Pb, and U) elements as set out by the ATSDR’s substance priority list, and compared the elemental distributions and concentrations in zooplankton from the three currents. There were differences in concentrations of some elements between the three currents – each current therefore had distinguishable but subtle compositions of metals in their zooplankton. For many of the essential elements, it seemed as if physiological regulation by the zooplankton was adequate in maintaining a relatively homogenous distribution of concentrations, especially towards sites located distant from the African continent. I found that V, Cr, As, Se, Hg, Ni, and U concentrations in zooplankton from some sites approached and exceeded the concentrations found in other studies. To the best of our knowledge, Ni had the second highest recorded concentration. With high concentrations from “hot spots”, a certain amount of concern is warranted. Geographically, a number of “hot spots” were found, mainly associated with harbours and river mouths of the MC and EACC, and some in shallow areas near the African mainland. However, there were differences in concentrations between these “hot spots”. There were also indications of volcanic contributions of metals in zooplankton collected from the SEC. I cannot conclude definitively that increased concentrations of metals in zooplankton from the EACC, MC, and SEC “hot spots” were solely from terrestrial sources or linked with pollution. The “hot spots” do suggest localised impacts that were not natural, apart from volcanic activity. Factors such as currents, upwelling, atmospheric deposition and primary productivity can affect the concentrations of metals in zooplankton. Currents close to the coast, eddies, and seasonal effects can influence the stratification (mixing) of nutrients, zooplankton, and metals in the water column, and therefore concentrations in zooplankton. I therefore recommend further research into phytoplankton and zooplankton from the WIO as well as research into water and sediment concentrations, especially near “hot spots”, as this will more closely identify different sources of metals and indicate areas to reduce metal pollution. My research has established a baseline against which future studies can measure changes linked to pollution, and identified certain metals and locations that would warrant closer attention