NUTRIENT DYNAMICS IN AMAZON SHELF WATERS - RESULTS FROM AMASSEDS

Four hydrographic cruises were conducted on the Amazon shelf as part o f the AMASSEDS field program. During each cruise, approximately 55 sta tions were occupied and nutrients, as well as other hydrographic param eters, were measured. The results of this time series sampling program indicate that the nutrient concentrations in the riverine end-member (silicate = 144 mu mol kg(-1), phosphate = 0.7 mu mol kg(-1), nitrate = 16 mu mol kg(-1), ammonium = 0.4 mu mol kg(-1), and urea = 0.9 mu mo l kg(-1)) remain relatively constant, despite a two-fold seasonal vari ation in river water discharge rate. Of the major nutrients (nitrate, phosphate, ammonium and silicate), nitrate shows the greatest seasonal change in riverine end-member concentration with a high value (23 mu mol kg(-1)) during the March cruise (rising river discharge) and a low value (12 mu mol kg(-1) during the November cruise (falling river dis charge). Nitrate is the dominant nutrient form of inorganic nitrogen t hroughout most of the river/ocean mixing zone, however, in the outersh elf area, where nitrate has been depleted by biological production, th is nutrient occurs at concentrations comparable to the other nitrogen species (ammonium, nitrite and urea), which are at levels <1 mu mol kg (-1). Nearshore, high turbidity inhibits phytoplankton production beca use of light limitation, whereas on the outershelf, nitrate appears to be limiting growth more than silicate or phosphate. Nutrient uptake w as observed during all four cruises, however, nearly all of this produ ction must be regenerated in shelf bottom waters, because very little of the biogenic materials are buried in the seabed (silicate burial <4 % of flux to algal blooms; similar to 10% burial of biologically avail able inorganic nitrogen reaching the river/ocean mixing zone; and <3% burial of phosphate flux to shelf environment). Clearly the Amazon she lf is not an efficient nutrient trap. Initial estimates of primary pro duction on the Amazon shelf suggest that algal blooms are sustained by regeneration to a large extent (up to 83%, 69% and 59% for N, P and S i, respectively) as well as by riverine and upwelled sources. Nutrient budget calculations have been used to establish the dominant external source of nutrients to the algal blooms occurring on the outer shelf. Based on flux core measurements, diffusive nutrient fluxes from Amazo n shelf sediments are very low relative to riverine supply rates (sili cate flux out = 1.3% of riverine flux, the nitrate plus ammonium flux is essentially zero, and the phosphate seabed flux shows removal of si milar to 2% of the riverine flux). Inventories of naturally occurring Pb-210 were used to estimate the onshore flow of subsurface water onto the Amazon shelf. The radiochemical data indicate that the flux of wa ter onto the shelf may be as much as five to ten times greater than th e annual flow of the Amazon River. The nutrient Aux from this shorewar d movement of ocean water (originating at a depth of 60-100 m water de pth) accounts for about 80% of the externally supplied ammonium, 52% o f the externally supplied phosphate, 38% of the externally supplied ni trate, and 17% of the externally supplied silicate reaching the outer shelf, with the remainder of the nutrient fluxes coming from the river . Therefore, the outershelf algal blooms are supported to a significan t extent by the shoreward flux of nutrients from offshore, subsurface waters.