Riverine and atmospheric inputs

Migon, C. 1993. Riverine and atmospheric inputs of heavy metals to the Ligurian Sea. Sci. Total Environ. 138 289-299. 

In Fig. 1.1 we already encountered a scheme of some of the chemical, biological, and physical processes which regulate the concentration of trace elements in the water column of lakes and oceans. When these trace elements are introduced into a lake by riverine and atmospheric input, they interact... 

Dissolved Cd concentrations increased slightly with a positive trend of 6.4%/year (0.0074nmol/(kg year)) between 1993 and 1999, which could be a result of riverine and atmospheric inputs (Fig. 13.5). This result was supported by the estimations of Dippner and Pohl (2004) using EOFs (empirical orthogonal functions). A positive trend on the order of one third of the standard deviation per year for Cdj.ss was calculated by using surface data for the whole Baltic Proper. 

Table 1 Estimates of the riverine and atmospheric input of some metals to Chesapeake Bay...

The riverine and atmospheric inputs to the oceans appear to be about equal but these fluxes are poorly known. With a total input of 10 mol year and an oceanic inventory of 10 mol, the average oceanic residence time of Cd is 10,000 years [18]. The bulk of the cadmium in seawater is in the dissolved phase and only a tiny fraction in suspended solids. The particulate concentration is in the range 0.04-4 pM, corresponding roughly to a concentration in the solid phase of 0.1-1 (jumol Cd mol C (Fig. 4) [19]. 

On shorter time scales, both riverine and atmospheric inputs can be highly episodic and are innately coupled, i.e., large precipitation events typically result in elevated stream transport. It has been shown (4) that the most prominent 10% of the precipitation events account for one third of the annual N wet deposition at Lewes. Similarly, for dry deposition, periods of high turbulence can lead to episodes of intensified deposition. Due to the differences between deposition velocity as a function of wind speed, the dry deposition associated with a wind speed of 20 m/s for 10 minutes a day would be equivalent to the deposition associated with an entire week at an average wind speed of 5 m/s (18). Such episodic behavior may have major implications in terms of ecosystem response, which in many instances may be more important than the cumulative loading. 

The ocean receives Cd mobilized from the crust through riverine and atmospheric input. These fluxes are poorly constrained at present but given an ocean Cd inventory of 10 ° g, the residence time of Cd is similar to biologically utilized elements and approaches lO years [70]. The predominant form of Cd in the ocean is in the dissolved phase with concentrations ranging from 1 to 1000 pmol kg [76-78]. The vertical distribution of Cd in the oceanic water column resembles profiles of phytoplankton nutrients, with minimum concentrations at the surface that increase to maximum values in the main thermocline and remain relatively constant from there to the ocean bottom (Figure 2) [76-78]. Particulate Cd concentrations are significantly lower and fall between 0.04 and 4 pmol kg and are, conversely, maximal in surface waters [79]. This distribution reflects the uptake of Cd by photosynthetic plankton at the surface and the sinking and subsequent decomposition of particulate matter in the water colunrn. 

It has been estimated that riverine and atmospheric N inputs in the North Atlantic Ocean basin are at least equal to and may exceed N inputs by biological N2 fixation (Howarth et al., 1996 Paerl and WhitaU, 1999). Therefore, our understanding of marine eutrophication dynamics needs to consider a range of scales including ecosystem, watershed, regional and global. 

Fixed N is also transferred from terrestrial to marine ecosystems by riverine and atmospheric vectors, in roughly equal parts (GaUoway et al, 2004). This probably contributed no more than one quarter, and perhaps less than one eighth, of the input flux of fixed N to the pre-industrial ocean (Galloway et al, 2004). Although it has been suggested that the of terrestrial inputs sums to near 0%o (Brandes and... 

Besides the particle-reactive character of this element, which leads to a rapid export and a short residence time of 0.5-1.3 years (Section 13.5), these findings are consistent with the worldwide decrease in the use of leaded gasoline, but they are in contrast to elevated riverine and atmospheric Pb inputs, according to the HELCOM pollution load compilation as shown in Table 13.2. Below the halocline, a negative Pbdiss trend of 15%/year ( 0.0142 nmol/(kg year)) was estimated for the period between 1995 and 2000. 

To answer the question how external atmospheric and riverine trace metal inputs are affected by the redox vaiiability of the system, the total external fluxes into the Gotland Basin have been estimated from the sum of riverine input and atmospheric input according to HELCOM (1998). They correspond with 31 nmol/m d for Pb, 3.0nmol/m2d for Cd, 1008 nmol/m d for Zn, and 290nmol/m d for Cu as shown in Table 13.5. 

In coastal seawater such as the Western Mediterranean basin, soil-derived particles originated from arid areas (in this case the Sahara). The atmospheric flux of anthropogenic trace metals, however, was dominated by aerosols from industrialized regions of Western Europe. Volcanic activity (Mount Etna) contributes selenium. The atmospheric input of Cr, Hg, Pb, and Zn into the Western Mediterranean basin is of the same order of magnitude as the riverine and coastal inputs of these components (Arnold et al. 1983). For the southern bight of the North Sea, estimates even indicate a predominance of the atmospheric input of... 

The mixture of organic constituents in the marine environment is extremely complex. Their origin is partly terrigenuous many compounds are produced in the marine environment itself. The different sources and their relative importance for the complexation of trace metals in estuaries, coastal seas and open ocean are riverine input, runoff from the coastal zone, resuspension, import of water mases, atmospheric input and in situ biological production. 

The atmospheric input of trace metals to the oceans has now been recognised to be of considerable importance to the marine system. Riverine sources have, for many years, been thought to be the dominant input route for metals into the oceans. Recent studies show, however, that these terrestrial fluxes may be equalled or even exceeded by inputs from the atmosphere. Thus, because of the importance of atmospheric input to the oceans, many of the examples considered here will focus on metal speciation in remote marine areas. In Table 7.1, average metal values in aerosols and rain at several sites are reported to show the range of concentrations of interest. 

Direct comparison of the atmospheric and riverine modes of input to the oceans can be misleading because of differences which act to maximise the importance of the atmospheric input route ... 

Nixon et al. (1996) have compiled N and P data for many of the watersheds and estuaries feeding the North Atlantic Ocean. In many of the watersheds of eastern North America, the input of N from anthropogenic atmospheric supply and sewage is a considerable fraction of the total input. This is not the case for the Amazon shelf, where atmospheric and sewage inputs are small relative to riverine input and upwelling. For the embayments and estuaries of the North Atlantic, Nixon et al. (1996) observed a significant correlation between %P exported and the mean freshwater replacement time. During the AmasSeds cruises the freshwater... 

The other Nr sources to the ocean are riverine injection and atmospheric deposition. Over the last few decades, riverine inputs to the ocean have ranged from 13 TgN yr to 76 TgN yr with the more recent estimates 40-50 TgN yr (Table 4). These inputs to coastal regions have significantly altered the associated ecosystems (NRC, 2000 Rabalais, 2002). 

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