Metal concentrations in seaweeds from KwaZulu-Natal, South Africa-a first report

A survey of concentrations of selected metals in some common seaweeds from the KwaZulu-Natal coast was conducted. Samples of 40 seaweeds were collected from Palm Beach, Isipingo Beach and Mission Rocks and analysed for metals by X-ray fluorescence. High metal concentrations were found in a number of the seaweeds examined. Slypocau/on funicu/are (Phaeophyta) and Osmundaria serrala (Rhodophyta) showed high levels of a wide range of metals and are recommended for further study as indicator species for metals in the marine environment of the KwaZuJu-Natal coast.


Introduction
Macroalgae are reportedly reliable indicators of metals in seawater because of their ability to accumulate these elements (Saenko ef al. 1976;Zindge ef al. 1976;Agadi ef al. 1978;Philips 1980;Ho 1988;Ferletta el at. 1996). Seaweeds have several intrinsic advantages which make them suitable indicators in the marine environmen t, viz. they are sessile and can be used to monitor changes in metal levels over time, their size makes them readily identifiable and they are easily handled (Lev ine 1984). For a species to be a good indicator, it must be a bio-accumulator and its population size should be adequate to susta in regular sampling.
The lite rature reports s ix methods for metal analys is in seaweeds: atomic absorpti on spect rophotometry (G uven el al. 1992; Molloy & Hills 1996), electron microprobe X-ray ana lys is (Chung & Lee 1989), thermal neutron activation analysis (GOven ef al. 1992), micro-PI XE (proton induced X-ray emission) imaging (Weiersbye et al. 1996), instrumental Neuron Activation analysis (INA A) (Vasquez & Guerra 1996) and X-ray fluorescence (XRF) (Bannatyne 1995). Metal concentrations in seaweeds from different parts of the world have been reported (Bryan & Hummerstone 1973, 1977Fuge & James 1974;Haug ef al. 1974; Morris & Bale 1975;Foster 1976;Eide ef al. 1980;Bryan 1983;Forsberg el al. 1988). However, no such data are available for South Afri can seaweeds. Due to the growing trends of informal settlement, urbanisation and industrialisation on the South A frican coastline, metal pollution of the marine environment is lik ely to increase and analys is of metal content in seaweeds may provide a useful monitoring too l for assess ing metal poll uti on leve ls. Reservations on the validity of seaweeds as bioindicators of metal levels in the environm ent have been expressed by Brown ( 1997).
Thi s study examined the levels of metals found in common ly occu rring seaweeds from Palm Beach (30058'S~ 30 o I7.5'E). a residential area su rrounded by sugar cane farming, lsipingo Beach (29°58'S; 32°58'E) a relatively highly industrialised area, and Mission Rocks (28 0 1'S; 32°30'E), a low impact coastal reserve surrounded by a national park, in KwaZulu-Natal, South Africa. This paper is the first report to use XRF to study the levels of metal s in seaweeds from KwaZulu-Natal.

Materials and Methods Seaweed sample coliection
Seasonally and locally abundant seaweeds were collected from the intertidal zone of Palm Beach, Isipingo Beach and Mission Rocks, KwaZulu-Natal, on the cast coast of So uth Africa in April 1995. Several large individual plan ts were co llected in the lield and pooled to provide a sample. Three replicate samples were taken . These were ai r-dried and sand, crustaceans and epiphytes were removed.

X-ray fluorescence (XRF)
The clean air-dried samples were milled to a particle size of 75 !Jill in a carbon -steel geological mill (TS250 grinding mill , Dickie and Stockier, Johannesburg, South Africa). Following standard methods, six drops of 'liqu id glue' [2% solution of mowiol (Hoechst, Krugersdorr. South Africa)} were mixed with 5 ml of each pow· dered seaw(:cd and compressed in a peliet press. five ml boric acid (Saarchem. Krugersdorp, South Africa) was added and the peliet \vas again compressed to form a bilayered pellet. Three pellets of each sample were prepared. Each bilayercd peliet was subjected to X·ray tluon::scence (Philips PW 1400, Hol land) in order to determine the levels of metals (Tables  I and 2). Cadmium and mercury could not be detected due to the nature of the detection tube used in this study. A multifactorial analysis of variance of the means, at the 95% confidence level, was used to compare metal levels among seav\I·eeds.

Resu lts and Discuss ion Baseline values and variation of metal levels in different seaweeds
All seaweeds collected from KwaZu lu-Natal contained detecta· ble levels of various metals. The 10 seaweeds which showed the highest metal content are listed in Table 1. Table 2 lists the remaining seaweeds analysed in this study. There were no significant differences in the metal levels between seaweed species of the same genus co ll ected at the same site (e.g. Amphiroa bowerbankii and A. ephedraea; Caulerpa filiformis and C racemosa; Chaetomorpha antennina and Ch. !inum) (Tables 1 and 2). However, metal ion levels varied greatly among seaweed species (Tab les I and 2). This trend is in agreement with reports on Malaysian seaweeds (Ramachandran et al. 1994).
Nb, Co, Y, Mo, V, Rb, Zr and Ni concentrations in the tissues of seaweeds elsewhere in the world are not reported in the litera· ture and are presented here for the first time (Tables 1 and 2). This high concentration ofPb in the rhodophytes is similar to the findings of Ramachandran e/ al. (1994) and is reportedly ascribed to the phycocolloid, carrageenan in these algae which have a strong affinity for Pb.

Conctusions
This study is the first report of metal levels to be found in some of the commonly occurring macroalgae from the coast of Kwa-Zulu-Natal. The algae listed in Tables I and 2 occu r in relative abu nd ance in KwaZulu-Natal. fo r 1110St part of the year and have the demonstrable ability to accumulate a range of metals. Th e two seaweeds that contained the highest levels of the majority of the metals tested in thi s study, were Stypocaulonfuniculare (Co, Cr, Ni. Y , Y and Zr), and Osmundaria serrata (As, Nb, Pb and Rb). It is suggested that these two seaweeds warrant further study as bio-ind icators of meta l leve ls in Sout h African inshore waters.