What proportion of riverine nutrients reaches the open ocean?

Globally, rivers deliver significant quantities of nitrogen (N) and phosphorus (P) to the coastal ocean each year. Currently, there are no viable estimates of how much of this N and P escapes biogeochemical processing on the shelf to be exported to the open ocean; most models of N and P cycling assume that either all or none of the riverine nutrients reach the open ocean. We address this problem by using a simple mechanistic model of how a low-salinity plume behaves outside an estuary mouth. The model results in a global map of riverine water residence times on the shelf, typically a few weeks at low latitudes and up to a year at higher latitudes, which agrees well with observations. We combine the map of plume residence times on the shelf with empirical relationships that link residence time to the proportions of dissolved inorganic N (DIN) and P (DIP) exported and use a database of riverine nutrient loads to estimate the global distribution of riverine DIN and DIP supplied to the open ocean. We estimate that 75% of DIN and 80% of DIP reaches the open ocean. Ignoring processing within estuaries yields annual totals of 17 Tg DIN and 1.2 Tg DIP reaching the open ocean. For DIN this supply is about 50% of that supplied via atmospheric deposition, with significant east-west contrasts across the main ocean basins. The main sources of uncertainty are exchange rates across the shelf break and the empirical relationships between nutrient processing and plume residence time

Has the role of estuaries as sources or sinks of dissolved inorganic phosphorus changed over time? Results of a Kd study

There are literature reports suggesting that some estuaries are sinks for dissolved inorganic phosphorus (DIP) whilst other estuaries appear to be sources of DIP. Here a simple Kd model is presented that is able to rationalize these disparate patterns of behaviour. This model suggests that riverine DIP levels are an important regulator of DIP behaviour in estuaries. DIP concentrations have increased in many rivers over the last 50 years as a result of human activity. The model results presented suggest that increases in riverine DIP concentrations from 5 μM, consistent with documented changes in some systems over the last 50 years, can change estuaries from being sources to sinks for DIP. The model results demonstrate that suspended solid concentrations in estuaries (based on simulations over the range 100–2000 mg 1−1) are also an important regulator of DIP behaviour and hence the modification of suspended solids via dredging and land reclamation activities in estuaries can alter their DIP removal capacity

Nutrients in estuaries

Nitrogen and phosphorus loading in rivers has increased considerably as a result of human activity. Silicon loads have been less impacted. However, extrapolating these increased loads in rivers through to coastal waters is not straightforward because of the intense nutrient cycling that can take place in estuaries. In this article we review the inputs to estuaries, methods of calculating fluxes through estuaries to coastal waters, and the processes that give rise to the intense cycling within estuaries. Although all the nutrients are intimately linked through their role in primary production, their cycling processes within estuaries are markedly different and hence estuarine cycling can not only change total nutrient loads, but also modify the ratios of one nutrient to another. These modifications of nutrient ratios may have important implications for both the extent and the number of species involved in primary productivity in coastal waters. Many estuaries are rather turbid, which limits the extent of primary productivity and hence the impact of this process on nutrient cycling. Primary productivity is probably the dominant process affecting dissolved silicon (Si) fluxes, and hence the modification of Si fluxes may be limited. The high turbidity will promote particle–water exchange reactions, which are particularly important for phosphorus (P) cycling. The high turbidity is also associated with net sedimentation in most estuaries and the sedimented material is rich in organic matter derived from riverine, marine and estuarine sources. The bacterial degradation of this organic matter drives a series of redox reactions which have a major impact on nitrogen (N) and P cycling. Denitrification can be a major sink for nitrate in estuaries, helping to attenuate its impact on coastal ecology, but the concomitant production of nitrous oxide may have a deleterious effect on the atmosphere. Iron(III) reduction in sediments can mobilize P bound to ferric oxyhydroxides in sediments and release this back into the water column. Considerable progress has been made on understanding nutrient cycling processes in individual estuaries, but we are still some time away from being able to generalize these results for other estuaries and hence effectively predict the present and future nutrient cycling in unstudied systems

High alkaline phosphatase activity in phosphate replete waters: The case of two macrotidal estuaries

The occurrence of alkaline phosphatase activity (APA) that hydrolyses organic phosphorus into phosphate (PO4) is commonly related to PO4 deficiency of oceanic, coastal and fresh waters. APA is almost never investigated in PO4-rich estuaries, since very low activities are expected to occur. As a consequence, microbial mineralization of organic phosphorus into PO4 has often been ignored in estuaries. In this study, we examined the importance of potential APA and the associated microbial dynamics in two estuaries, the Aulne and the Elorn (Northwestern France), presenting two different levels of PO4 concentrations. Unexpected high potential APA was observed in both estuaries. Values ranged from 50 to 506 nmol L−1 h−1, which range is usually found in very phosphorus-limited environments. High potential APA values were observed in the oligohaline zone (salinity 5–15) in spring and summer, corresponding to a PO4 peak and a maximum bacterial production of particle-attached bacteria. In all cases, high potential APA was associated with high suspended particulate matter and total particulate phosphorus. The low contribution of the 0.2–1 μm fraction to total APA, the strong correlation between particulate APA and bacterial biomass, and the close relationship between the production of particle-attached bacteria and APA, suggested that high potential APA is mainly due to particle-attached bacteria. These results suggest that the microbial mineralization of organic phosphorus may contribute to an estuarine PO4 production in spring and summer besides physicochemical processes