Seasonal changes in phytoplankton community structure in relation to physico-chemical factors in Loch Lomond,Scotland

Surface waters, sediments and the polychaete Nereis diversicolor were sampled in the ScheldtEstuary between 1990 and 1994. In surface watersparticulate Hg (Hg_P) concentrations ranged from350–1610 ng g^-1. They are essentiallycontrolled by physical mixing of polluted fluvialparticulates with relatively unpolluted marineparticulates, but unaffected by seasonal changes.Dissolved Hg species, on the other hand, show largeseasonal variations essentially controlled by theredox conditions in the estuary, as well as bybacterial and phytoplankton activity. Total dissolvedHg (Hg_TD) concentrations ranged from 0.5 to 3.0 ngl^-1 with 10 to 90% as reactive Hg. Highconcentrations of Hg_TD are found in the upperestuary in the winter and decrease rapidly withincreasing salinity. In summer Hg_TDconcentrations are low in the anoxic upper estuary andincrease as oxygen is restored in the estuary.Significant variations were observed in dissolvedMonomethyl Hg (MMHg) concentrations withconcentrations ranging from 0.01 to 0.120 ng l^-1 in the winter and 0.08 to 0.6 ng l^-1 in summerand autumn. Particulate MMHg ranged from 2 to 6 ngg^-1 in winter and from 4 to 10 ng g^-1 insummer and accounted for 20 to 80% of the total MMHg.Hg° concentrations ranged from 0.02 to 0.130 ngl^-1 and are higher in summer than in winter. Inthe lower estuary a positive correlation betweenHg° and phytoplankton pigments was observed. Sediments and the polychaete N. diversicolorwere sampled on the intertidal flat GrootBuitenschoor. Hg_{_T} concentrations in surfacesediments ranged from 144 to 1890 ng g^-1 and MMHgfrom 0.8 to 6 ng g^-1 accounting for 0.4 to 0.8%of the total mercury present. Both total Hg (Hg_T)and MMHg concentrations increased with increasedorganic matter content and anoxic conditions. On theother hand, accumulation of Hg_T and MMHg washigher in N. diversicolor living in coarse grainsandy sediments than in muddy sediments. MMHgconcentrations in N. diversicolor ranged from2.2 to 20.9 ng g^-1 accounting for an average of18% of the Hg_T. Seasonal variationssignificantly affected Hg speciation in sediments andN. diversicolor. Higher Hg_T concentrationswere found in the sediments in autumn and winter,whereas MMHg concentrations increased in spring andsummer. Likewise, higher MMHg concentrations were alsoobserved in N. diversicolor in spring andsummer. This revised version was published online in July 2006 with corrections to the Cover Date.