Acidification of airborne dust particles can dramatically increase the amount of bioavailable phosphorus (P) deposited on the surface ocean. Experiments were conducted to simulate atmospheric processes and determine the dissolution behavior of P compounds in dust and dust precursor soils. Acid dissolution occurs rapidly (seconds to minutes) and is controlled by the amount of H + ions present. For H + < 10 −4 mol/g of dust, 1-10% of the total P is dissolved, largely as a result of dissolution of surface-bound forms. At H + > 10 −4 mol/g of dust, the amount of P (and calcium) released has a direct proportionality to the amount of H + consumed until all inorganic P minerals are exhausted and the final pH remains acidic. Once dissolved, P will stay in solution due to slow precipitation kinetics. Dissolution of apatite-P (Ap-P), the major mineral phase in dust (79-96%), occurs whether calcium carbonate (calcite) is present or not, although the increase in dissolved P is greater if calcite is absent or if the particles are externally mixed. The system was modeled adequately as a simple mixture of Ap-P and calcite. P dissolves readily by acid processes in the atmosphere in contrast to iron, which dissolves more slowly and is subject to reprecipitation at cloud water pH. We show that acidification can increase bioavailable P deposition over large areas of the globe, and may explain much of the previously observed patterns of variability in leachable P in oceanic areas where primary productivity is limited by this nutrient (e.g., Mediterranean).atmospheric processing | ocean macronutrients | desert dusts

An intensive inar~culture system for the growth of sea bream has been developed which uses seaweed Ulva lactuca to remove d~ssolved nutr~ents ( N and P) from the circulating water This allows water to be safely recycled to the fish and results ~n a comparatively 'non-polluting' final effluent As with other marlculture systems, only -25% of the food-N was used for f~s h growth The damnant processes affecting N illthin the system were excietion by flsh of ammonia-N (30%), dissolved organlc mtrogen (DON) (30'X,) and faeces-N (1OU%) rapid transformation of DON to ammonia-N (16%) In the fish tank and uptake of aminonla-N (17 to 3 9 % ) by the seaweed (all percentages e x p~e s s e d relative to food input) N i t r~f~c a t~o n (10%) competed with the seaweed for the available a m m o n~a -N There was only min~mal uptake of mtrate by the seaweed Anoxic condltlons developed in the sedimentation tank Denitiification (5%) and bacterlal ammon~fication, ~ncludlng sulphate reduction, were observed The system showed only m n o r seasonahty, probably because the malor processes (food input and U lactuca uptake) had only a slight dependence on temperature Our quantitative understanding of the processes affecting N within the system was conf~rmed by a parallel study In which a computer simulation model was developed which was able to p r e d~c t well the observed changes In ammonia-N, total oxidised N and U lactuca growth There was a n excess of phosphorus In the food supplied It was removed by the seaweed (9 to 21 %) but to a lesser degree than ammonia-N The residual organlc matter in the s e d~m e n t a t~o n tank was ennched In P, probably because of the presence in the food of bone meal which is resistant to bacterlal decdv The final effluent d~s c h a r g e d contarned 20 to 27 % of the N supplied (and 39 to 47 % of the P) which IS less than half the N d~s c h a r g e d from conventional manculture technology

ABSTRACT. The rates of mineralization of organic-C and -N were measured m the sediments from 2 earthen fishponds in Eilat, Israel. The flux of the mineralized products (NH: and COz) across the sediment-water interface was also measured. It was concluded that the diffusional flux accounted for approximately 70 % of the flux; fish bioturbation was responsible for the remainder. The sediments were highly anaerobic and rates of SO:-reduction had a mean value of 70 mm01 m-2 d-l. Nitrification was absent, denitrification was insignificant and much of the sedimenting organic detritus was mineralized. This amounted to 140 mm01 C and 18 mm01 N m-' d-l, equivalent to a minimum input of > 600 g C m-2 yr-l.

The Cyprus Eddy, a warm-core eddy southeast of Cyprus, was sampled towards the end of an exceptionally cold winter in early March 1992, within 4 d of a storm and w~thin 24 h of an intrusion of cold air. Depth profiles of temperature, salinity and dissolved nutrients showed an active deep mixed layer from the surface to ca 500 m at the core of the eddy, while at the eddy boundaries the mixed layer extended only to 150 m. Microb~al populations were evenly distributed over the entire upper 500 m at the core station, as iiidicated by chlorophy!! and high performance liquid chromatography (HPLC)-determined pigment composition, by flow-cytometric analysis of the ultraphytoplankton, by direct counts of 4',6-diam~dino-2-phenylindole (DAP1)-stained bacter~a and %-thymidine measurements of bacterial activity. As far as we know, thls is the first deta~led description of the microbial populations in a warm-core eddy during the bloom season. The integrated water column chlorophyll content, 59 mg m-2 at the core and 45.5 mg m-2 at the boundary, was more than double the typical late autumn values, suggesting a bloom was occurring. Noticeably, this bloom was not delayed until the establishment of summer stratification as has been observed previously in warm-core eddies. While theoretical considerations based on the calculated critical depth at the core of about 300 m suggested that a bloom should not have occurred, our data jointly with previous data from the Cyprus Eddy support the hypothesis that interim periods of quiescence between mixlng events enable bloom development even when the mixing depth is greater than the critical depth. Added nutrients and dilution of grazers, both resulting from the deep mixing, probably contributed jointly to the enhanced productivity. Based on phytoplankton 11ght-shade adaptation features and cellular chlorophyll fluorescence per cell, we calculated that the rate of vertical mixing in the core was at least 30 m h-'