Nutritional quantity and quality of the particulate environment in April and July/August
4.4.8. Evaluation of particular fatty acids as biomarkers for individual microplankton groups In general, highly significant positive correlations between
the quantities of proposed indicator fatty acids (Table 1.1) and the biomasses of individual cell groups were found (Table 4.5), supporting the findings of previous field investigations (Kattner et al. 1983, Clauste et al. 1990, Skerratt et al. 1995, Hamm et al. 2001), and demonstrating that fatty acid analysis of particulate samples can provide qualitative information about the relative contributions of individual cell groups to the microplanktonic community (though see Hamm et al. 2001).
As discussed above, outside diatom bloom situations, the North Atlantic microplankton community is typically composed of small flagellated cells. Counter intuitively, a positive correlation between the 16:1(n-7)/16:0 ratio and the biomass of flagellates was found. However, although the biomasses of diatoms and flagellates were both greater in July/August, the relative increase in diatom biomass was much greater (700 %) than that of the flagellates (311 %), explaining why flagellates and the 16:1(n-7)/16:0 ratio were positively correlated. The biomass of diatoms was significantly correlated with the 16:1(n-7)/16:0 ratio and the quantities of 16:1(n-7) and 20:5(n-3), thus confirming the usefulness of these markers as indicators of
diatom abundance. However, since the ratio increased from 0.21 in April to only 0.45 in July/August, diatoms would not be expected to be a major component of either community. The C18 fatty acids, particularly 18:3(n-3) and 18:4(n-4), also proved to be useful biomarkers, and were significantly related to the abundance of both dinoflagellates and small flagellates. Additionally, 22:6(n-3) is also prominent in flagellated cells (Table 1.1) and despite some previous studies failing to find a correlation between this fatty acid and the biomasses of dinoflagellates and flagellates (Parrish et al. 1995, Reuss and Poulsen 2002), the data presented here showed significant correlations. Taxonomic distinction between flagellate groups using inverted microscopy was not possible. However, the fatty acid profiles can be tentatively used to provide such information. A significant positive correlation between 18:1(n-9) and the biomass of small flagellates (< 3.5 µm ESD) was also apparent, supporting previous field observations that have noted increases in the abundance of 18:1(n-9) during periods of flagellate (Phaeocystis sp.) predominance (Al-Hasan et al. 1990, Claustre et al. 1990, Skerratt et al. 1995, Tang et al. 2001, Reuss and Poulsen 2002). This positive correlation suggesting that at least a proportion of the small flagellates were indeed Haptophytes, and perhaps even single cells of Phaeocystis sp.. Independent samples taken above the Reykjanes Ridge during the same cruises (D262 and D264) support this idea, with Phaeocystis sp. dominating the microplankton biomass in April and contributing to the flagellate biomass in July/August (Irigoien et al. 2003). Chlorophytes may also have contributed to the flagellate community. The fatty acids characteristic of this group (16:4(n-1) and 18:3(n-3)) were both significantly correlated with the biomass of flagellates.
4.5. SUMMARY
Nutrient (N, Si, and P) concentrations in April were high, yet chlorophyll concentrations remained low. This suggested that at the time of sampling, the spring bloom had yet to occur. This was confirmed by examining satellite observations of chlorophyll a, which showed that elevated levels of chlorophyll were not present until June. The microplankton community was typical of a pre-bloom community in the North Atlantic, being heavily dominated by small flagellated organisms. This was confirmed by the abundance of the flagellate biomarker, 18:1(n-9) and the low 16:1(n-7)/16:0 ratio. In addition, microzooplankton also contributed significantly to
the microplankton community, typically representing > 20 % of the total microplankton biomass. In contrast, diatoms were scarce, contributing < 3 % to the daily community biomass. This was also reflected in the low quantities of the diatom biomarker, 16:1(n-7), which did not exceed > 5 % of the total fatty acid composition. The total quantities of fatty acids in the samples collected at the stations in April were low, and EPA and DHA together represented < 8 % of the fatty acid composition.
By July/August, the nutrient concentrations were much lower, suggesting a large draw down in the interim period. Although the prominence of diatoms had increased, the microplankton community was again dominated by flagellates. These observations were reflected by the abundance of diatom and flagellate biomarkers in the seston. Microzooplankton were also a prominent feature of the microplankton, contributing between 10 and 39 % to the community biomass. The stations sampled in July/August were representative of a post-bloom microplankton community. Despite being characteristically small, all the cells enumerated in April and July/August were considered to be potential prey items for C. finmarchicus.
Significant inter-seasonal differences in the microplankton communities were found. These were driven by the relative abundance of the individual flagellate groups identified, and also the abundance of diatoms. Similarly, inter-seasonal differences in the fatty acid compositions were significant. The PUFAs 20:4(n-6) and 22:6(n-3) were responsible for much of the observed differences, being greater in July/August. Furthermore, quantities of POC, PON, EPA, DHA and microplankton biomass were all significantly greater in the samples collected in July/August. Large quantities of detritus were observed in the microplankton samples from both seasons. This explains why < 20 % and < 40 % of the POC was attributable to microplankton biomass in April and July/August respectively. The detrital biomarker, 18:0, indicated that although absolute quantities were greater in July/August, the relative abundance of detritus was greater in April. This was confirmed by the higher POC:PON, POC:cell biomass and POC:PUFA ratios in April. Calanus is assumed not to ingest detritus, therefore POC data provides limited information about the food available to the copepods during the incubations. Cell biomass, determined by inverted microscopy, was considered to be a much more reliable estimate of the available food. Polyunsaturated fatty acids are primarily associated with viable microplankton, therefore cell biomass:EPA and cell biomass:DHA ratios were used
to assess the relative quality of the food. These ratios were much greater in July/August, indicating that the microplankton sampled after the spring bloom had a greater nutritional value relative to that sampled before the bloom.
Individual fatty acids and fatty acid ratios were significantly correlated with particular algal classes. The 16:1(n-7)/16:0 ratio appears useful in determining the relative contributions of diatoms and flagellates to the microplankton community. Additionally, the C18 fatty acids, 18:3(n-3) and 18:4(n-3), were useful indicators for the presence of flagellated cells. However, the understanding of particulate fatty acid data was greatly enhanced with the addition of cell biomass data, as determined by inverted microscopy.