Mesoscale feature

Ingram, R.G, and El-Sabh, M.I. (1990) Fronts and mesoscale features in the St. Lawrence Estuary. In Oceanography of a Large-Scale Estuarine System, The St. Lawrence (El-Sabh, M.I., and Silverberg, N, eds.), pp. 71-93, Springer-Verlag, New York. 

FIA is readily adaptable to the determination of dissolved nutrients in seawater. Methods for the determination of nitrate 11, 12), phosphate (13), and silicate (14) in seawater have been developed. We have used FIA methods at sea and they have proven to be quite reliable. Because high sampling rates are possible with FIA (i5), it is well-suited for oceanographic investigations of small-scale and mesoscale features in the ocean. It is particularly well-suited to applications where it is interfaced with the effluent of a submersible pumping system. 

These analyses, combined with data from in situ sensors, provide information on small-scale and mesoscale features not otherwise available. Signals in the sample stream are modified by passage through the hose. This modification and time delays introduced by analysis must be considered in sampling strategy and data management approaches. This system has been used to determine nutrients, in vivo fluorescence, and temperature in a warm core ring. Examples of the results are provided the fluorescence, temperature, and nitrate distributions show considerable independence. 

Continuous underway sampling using a deep-towed pump and a hose-cable can examine chemical patterns in the ocean that are nearly impossible to describe by other techniques. Growing realization of the importance of mesoscale features and oceanic fronts makes use of such sampling tools an essential part of oceanography. Effective data management must be an indispensable part of any deep-towed sampling system. 

Like chlorophyll, bacterial biomass was also shown to be uniformly distributed horizontally throughout large areas of the Levantine Basin with the exception of mesoscale features where the pattern was usually altered. 

Effects of mesoscale features on nutrient and chlorophyll distribution and phytoplankton productivity... 

As has been noted above, the Eastern Mediterranean is characterised by many eddies and jets (POEM, 1992). Indeed there are almost no areas of the basin which are not part of some mesoscale feature or other (Fig. 4.3). Yet the nutrient distribution (Kress Herat, 2001) and many of the plankton features such as bacterial abundance and activity and chlorophyll content (Yacobi etal., 1995) seem to be nearly constant across large parts of the basin except for those locations where they intersect major and persistent mesoscale features (Fig. 4.5). Under those circumstances major changes in nutrient distribution and productivity can be seen. The Rhodes Gyre and the Cyprus Eddy (aka Shikmona Gyre) are permanent features which always have an effect on the local biogeochemistry and have been studied in some detail. 

Table 4.2 Table showing seasonal variations in the phytoplankton biomass due to mesoscale features in the Levantine Basin... 

By March the seasonal thermocline would have reappeared and the characteristic summer conditions begin to reestablish themselves initially in the southern Levantine Basin. With the initiation of the bloom, mesoscale features control the details of the re-establishment of summer conditions. Eddies can still be clearly seen as areas of higher biomass in the southern Levantine, particularly in March. Summer conditions with extremely low productivity in the surface layers, and a DCM is fully established by May and reach their peak between June and August, when all of the southern Levantine has extremely low chlorophyll levels in the surface layers. It is important to remember that the satellite only sees the phytoplankton in the uppermost layers of the system. Once the seasonal thermocline is developed, the system is characterised by a deep chlorophyll maximum. Thus the drastic changes in chlorophyll shown by these images are in reality less marked. 

The phytoplankton community in the offshore region is dominated by pico- and nano-plankton, with microplankton and eukaryotes only important in coastal regions and upwelling areas associated with mesoscale features such as the Rhodes Gyre. Heterotrophic bacteria are an important component of this nutrient depleted system. Both depth integrated chlorophyll a and primary productivity levels are extremely low, characteristic of ultra-oligotrophic systems. 

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