TRACING THE NATURAL AND ANTHROPOGENIC INFLUENCE ON THE CHEMISTRY OF ESTUARINE MACROALGAE, AND THE IMPLICATIONS IN RESPECT TO HUMAN CONSUMPTION: INSIGHTS FROM THE FIRTH OF FORTH AND FORTH ESTUARY, SCOTLAND (Invited Presentation)

Macroalgae (seaweed) can be used as an environmental indicator as it has previously been shown to reflect the chemistry of the water in which it lives. Here, we present the trace element chemistry (As, I, Hg, Zn, Pb, Cu, Ni, U, Ag, Cd, Co, Re and Os) of five different seaweed species from the Firth of Forth and Forth Estuary in Scotland (Forth estuary). The overall trend in elemental abundance (Os << Re < Ag < U < Cd < Co < Ni < Pb < Cu < As < Zn << I ppm dry weight) and changes in abundance along the estuary (increase in As, I, Cd, U, Re, Os; decrease in Pb, Cu; mid-estuary peak in Ni, Zn) are controlled by a number of factors. These include: intra- and inter-macroalgae species differences such as affinity for uptake, age, and exposure to the atmosphere, and also salinity and freshwater–seawater mixing. Mixing is considered to play a role in many of the elemental abundance trends along the length of the Forth estuary, with mixing between end members of freshwater riverine inputs and saltwater inputs from the North Sea. This is thought to have the largest effect on As, I, Pb, Cu, Cd and U abundances. Non-conservative mixing processes resulting from in-estuary sources, such as natural and anthropogenic additions from sewage, industry and the surrounding geology, also play a role, affecting Zn, Ni, Ag, Co, Re and Os abundances.

Given the potential health benefits, the consumption of macroalgae is becoming increasingly popular. For example the species, L. digitata, is harvested close to one of the studied locations. Abundances of iodine (67–5061 ppm), lead (0.047–4.1 ppm) and cadmium (0.006–0.93 ppm) in the Forth estuary macroalgae are found to be at relatively safe levels with regards to human consumption. However, total arsenic (12–135 ppm) is very high and toxic inorganic As (0–67 ppm) abundances in many samples exceed the American (3 ppm) and Australian (1 ppm) regulations for inorganic As in macroalgae. Abundances of some trace metals, I and As can be greatly reduced through soaking and cooking of the macroalgae. Cooking, in most cases, reduces the inorganic As levels in the macroalgae to within the American and Australian safe regulation limits. However, if the macroalgae is used to cook soups (e.g., Dashi), this creates further problems as the elements leach out into the soup that is then consumed.