Open oceans

We can view the open ocean as the largest of all endorheic 'lakes' - a huge basin of water supplied by the world's rivers and losing water only by evaporation. Its great size, in comparison to the input from rain and rivers, leads to a remarkably constant chemical composition.

We considered biologically mediated transformations of carbon in terrestrial ecosystems in Section 18.2.3. Figure 18.15 illustrates the same thing but for the open ocean. The main transformers of dissolved inorganic carbon (essentially CO2) are the small phytoplankton, which recycle CO2 in the euphotic zone, and the larger plankton, which generate the majority of the carbon flux in particulate and dissolved organic form to the deep ocean floor. Figure 18.16 shows that, in general, only a small proportion of carbon fixed near the

Year Year

Figure 18.13 Dissolved silicate (DSi) concentrations at the river mouths of (a) the nondammed River Kalixalven and (b) the dammed River Lulealven. (Humborg et al., 2002).

18.3.4 Continental coastal regions of oceans are influenced by their terrestrial catchment areas...

... and local upwelling the open ocean: an important role for plankton ...

(a) Urunga (b) Diamond Head (c) Point Stephens

Figure 18.14 Contours of nitrate concentration during upwelling events along the New South Wales coast at: (a) Urunga (wind-driven), (b) Diamond Head (encroachment-driven), and (c) Point Stephens (separation-driven). The bottom graph in each case shows the mean nitrate concentrations that can be taken as characteristic of these sites in the absence of an upwelling event. Maximum concentration is 10 |lmol l-1. The contour interval is 1 or 2 |lmol l-1 and the thick orange line represents 8 |lmol l-1. (After Roughan & Middleton, 2002.)

Figure 18.14 Contours of nitrate concentration during upwelling events along the New South Wales coast at: (a) Urunga (wind-driven), (b) Diamond Head (encroachment-driven), and (c) Point Stephens (separation-driven). The bottom graph in each case shows the mean nitrate concentrations that can be taken as characteristic of these sites in the absence of an upwelling event. Maximum concentration is 10 |lmol l-1. The contour interval is 1 or 2 |lmol l-1 and the thick orange line represents 8 |lmol l-1. (After Roughan & Middleton, 2002.)

surface finds its way to the ocean bed. What reaches the ocean floor is consumed by the deep-sea biota, some is remineralized into dissolved organic form by decomposers, and a small proportion becomes buried in the sediment.

Just as we saw in terrestrial ecosystems, marked seasonal and interannual differences in nutrient flux and availability can be detected in the deep ocean. Thus, Figure 18.17a shows how chlorophyll a concentrations varied during the spring bloom at a site in the North Atlantic, reflecting a succession of dominant phytoplankton species. Large diatoms bloomed first, consuming almost all the available silicate (Figure 18.17b). Subsequently, a bloom of small flagellates used up the remaining nitrate. Over a longer timescale, a remarkable shift in the relative abundance of organic nitrogen and phosporus has been witnessed in the North Pacific.

The ocean has traditionally been viewed as nitrogen-limited, but, when nitrogen limitation is extreme, nitrogen-fixing taxa such as Trichodesmium spp. grow over large areas and bring into play the inexhaustible pool of dissolved N2 in the ocean. This has led to a decade-long shift in the N : P ratio in suspended partic-ulate organic matter (Figure 18.17c). Under these circumstances, phosphorus, iron or some other nutrient will eventually limit productivity.

About 30% of the world's oceans have long been known to have low productivity despite high concentrations of nitrate. The hypothesis that this paradox was due to the iron limitation of phytoplankton productivity has been tested in locations as different as the eastern equatorial Pacific and the open polar Southern Ocean (Boyd, 2002). Large infusions of dissolved iron

... which may follow a seasonal pattern iron as a factor limiting ocean primary productivity?

Figure 18.15 Biologically mediated transformations of carbon in the open ocean. (After Fasham et al., 2001.)

Equatorial Pacific

A

Arabian Sea: Aug-Sept

Sargasso Sea

0

Polar regions

A

Arabian Sea: Jan-Jul

O

North Atlantic

Figure 18.16 Relationship between the export of particulate organic carbon (POC) to the ocean depths, recorded at 100 m, and ocean primary productivity in the world's oceans. (After Buesseler, 1998.)

at sites in each ocean led in both cases to dramatic increases in primary productivity and decreases in nitrate and silicate, as these were taken up during algal production (the results are expressed as nitrate removal in Figure 18.18). Bacterial productivity tripled within a few days in both cases, and rates of herbivory by micrograzers (flagellates and ciliates) also increased, but less so in the polar situation (where dominance by a grazer-resistant, highly silicified diatom probably suppressed grazing). The metazoan community, dominated by copepods, showed relatively little change in either situation.

It is an intriguing thought that in places such as the eastern equatorial Pacific or polar Southern Ocean, blooms in productivity might sometimes be caused by long-distance wind transport of land-derived, iron-rich particles. This would mirror, but on a very different scale, the high productivity associated with inputs of land-derived, nutrient-rich water from rivers.

Figure 18.16 Relationship between the export of particulate organic carbon (POC) to the ocean depths, recorded at 100 m, and ocean primary productivity in the world's oceans. (After Buesseler, 1998.)

Figure 18.17 Patterns in (a) chlorophyll a and (b) silicate and nitrate concentrations during a spring bloom in the North Atlantic. Day number is days since January 1. (After Fasham et al., 2001.) (c) Shift in the ratio of N : P in suspended particulate matter measured in the North Pacific Gyre. (After Karl, 1999.)

o 110

ie 2

o 110

15 30 m

O Nitrate A Silicate

130 140

Day number

15 30 m

Jan July Jan July Jan July Jan July Jan July Jan July Jan July Jan July Jan July Jan July 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

3000 2500 2000 1500 1000 500 0

B.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

Time (days)

Figure 18.18 (a) Rates of depth-integrated net primary production (NPP) after iron addition at sites in the eastern equatorial Pacific Ocean (□) and polar Southern Ocean (•). (b) Nitrate removal during the time course of the two experiments. Note that silicate followed similar patterns. (After Boyd, 2002.)

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