North Pacific trades biome

The Nitrogen Cycle in the North Pacific Trades Biome An Evolving Paradigm... 

Figure 16.3 Microbial food web processes sustain the marine N-cycle in the North Pacific trades biome. Shown are (A) a schematic view of the various sources, transformations and sinks for key N pools and...

Figure 16.4 Map of the North Padfic Ocean basin showing several important features of the trades biome. (A) Dynamic topography of the sea surface in dyn-cm relative to 1000 dbar based on historical hydrographic observations. Arrows show the direction of geostrophic flow. From Wyrtki (1975). (B) Sea surface distributions of chlorophyll (mg m for the Pacific Ocean in 2003 from 15°S to 65°N latitude as derived from the AQUA MODIS satellite-based sensor system (4 km resolution). Superimposed on ocean color, in white contour lines, is the mean annual surface nitrate concentration (mmol NO m ) based on the World Ocean Atlas (2001) Ocean Climate Laboratory/NODC. Areas of high NOs (and presumably NOs flux) correspond to areas that are enriched in chlorophyll as a result of net plant growth. The North Pacific trades biome is the central region of low standing stocks of plants (<0.1 mg m blue-purple areas) and low ambient NOT concentrations (<1 mmol m ).

Figure 16.5 A conceptualized view of the new versus regenerated N model based on the classic work of Dugdale and Goering (1967). Shown are two contrasting marine ecosystems (Left) an upwelling habitat where allochthonous NO3 -supported new production dominates total primary productivity, and (Right) an open ocean habitat where locally produced NH4+-supported regenerated production dominates total primary productivity. New production-intensive biomes also support much greater export per unit area, usually in the form of sinking particulate matter, than remineralization-intensive systems like the North Pacific trades biome.

Table 16.2 List of Selected Key Publications, Programs, Expeditions and Intellectual Breakthroughs that have Contributed to our Current Understanding of the Marine N-cycle in the North Pacific Trades Biome... 

The concentrations and dynamics of the near-surface DON and PON pools have been studied at Station ALOHA since 1988. Church et al (2002) reported that the 0—175 m dissolved organic C (DOC) and DON (but not dissolved organic P) increased at rates of 303 and 33 mmol m year respectively, for the period 1993—1999. The accumulated DOM had a mean C N molar ratio of 27.5. By comparison, the C N ratio of isolated HMW-DOM in the North Pacific trades biome is 14—15, suggesting that the highly aged (based on C content) LMW-DOM is more carbon rich (Loh et al, 2004). However, the true C N ratio of the DON pool (as opposed to bulk DOM) is neither known nor easily determined because the DOC and DOC-N, DOC-N-P and DOC-P sub-pools cannot currently be separated. For example, the N content of HMW-DOM isolated from Station ALOHA varied between 0.95 and 1.69 wt% with no clear depth trends between 20 and 4000 m molecular analyses identified carbohydrate and amino acids as major compound classes (Sannigrahi et al, 2005). 

Several comprehensive studies of N assimilation in the North Pacific trades biome have been conducted over the past several decades. Gundersen and his colleagues (1974, 1976) were the first to estabhsh N2 fixation as a source of new N to the open ocean ecosystem, and concluded that it was a more important source of fixed N than wet deposition from the atmosphere (see Case Studies section). They also made measurements of the rates of nitrification, denitrification and assimilatory nitrate-reduction. These latter experiments involved the addition of fairly high concentrations of exogenous N substrates (NH4 , N02, NOa ) and extended incubations (days to months), so the rates reported must be viewed as potential rates at best. 

As one component of the decade-long VERTEX program, an oceanic time-series station (33°N, 139°W) was occupied for an 18-month period from October 1986 to May 1988. During this observation period, the site was visited on 7 occasions ( 90-day interval) for approximately 1 week per expedition to retrieve and redeploy a free-drifting sediment trap array, to collect water samples and to conduct experiments relevant to C- and N-cycle processes (Harrison et al., 1992 Knauer et al., 1990). The uptake and assimilation of NOa and NH4 substrates were measured during incubation experiments that were designed to assess, and correct for, isotope dilution of the added tracers. Photoautotrophic N assimilation was measured using the into protein method, described later in this section. Measurements were also made of the concentrations of NOs , NH4, DON, PON, total microbial biomass, autotrophic biomass, heterotrophic biomass, primary productivity and the export of particulate matter (Harrison et al, 1992). In many ways this was, at that time, the most comprehensive study of the marine N-cycle ever conducted in the North Pacific trades biome. 

At a station located in the North Pacific trades biome (18°N, 156°W), DiTuUio and Laws (1983) reported autotrophic N uptake rates of 7-11 pmol N m h for four samples coUectedin the euphotic zone. The integrated (0—120 m to the l%Ught level) rate of autotrophic N assimUation was 0.93 mmol N var h and the mean molar C N assimilation ratio (C assimilation was measured using " C) was 7.84 ( 0.23), compared... 

As mentioned previously, Prochlorococcus is the dominant phytoplankton group in the North Pacific trades biome. Recently, the fuU genome sequences of several representative Prochlorococcus ecotypes have been pubhshed (Dufresne et al, 2003 Rocap et al., 2003). It is important to point out that none of the three genomes sequenced contain nitrate reductase, the enzyme responsible for the reduction of NOs to N02, the hypothesized mechanism for the existence of the PNM layer. This is not to say that Prochlorococcus does not contribute to the PNM, rather that we have no evidence to date that they can utilize NOs . However, recent results suggest that a yet-to-be-isolated Prochlorococcus ecotype may contain nitrate reductase (Casey et al., 2007). Furthermore, the deep living/dark-adapted ecotype of Prochlorococcus, as weU as other microbes, can utihze N02 as a source of N for biosynthesis so the net effect of phytoplankton/microbe metabolism would be to erode, not to produce or sustain, the PNM. 

Dore et al. (2002) were the first to report euphotic zone depth-integrated measurements of the relative contributions of smaU (<10 pm) and large diazotrophs to total N2 fixation in the North Pacific trades biome. Based on N2 assimilation rate measurements from HOT cruises conducted in November 2000... 

DNA extraction and amplification efficiencies, and identification of the number of gene copies per cell among the disparate diazotroph groups. However, these gene-based techniques provide a quantitative approach to the important question of diazotroph diversity. Their initial field results opened a new window of observation for the study of N2 fixation in the North Pacific trades biome. 

In addition to NOa , delivery to the surface ocean of atmospheric NH4 is also potentially important though poorly documented for the North Pacific trades biome at the present time. Clarke and Porter (1993) reported significant NH4 concentrations in aerosols coincident with local enrichments in near-surface chlorophyll, suggesting a biogenic source or control. Estimates of the sea-to-air flux of NH4 ... 

---