Reliable maps of euphausiid abundance are difficult to generate for a number of reasons. Firstly, samples that can be used for qualitative purposes often cannot be used for quantitative purposes, because data on the volume filtered by the nets were not recorded. Secondly, as few cruises have focused specifically on euphausiids the methods of sample collection have frequently been inappropriate (see methods). Thirdly, there has been a lack of standardisation in the way the data that have been presented (e.g. wet weight, settled volume). Lastly, such maps should be based on estimates of annual mean abundance. These are often difficult to disentangle from seasonal and diel ranges, especially as they are often the product of cruises of opportunity and only provide a brief glimpse of patterns in time. Discussion here is, therefore, strictly confined to patterns of abundance at their crudest level.
Two basic patterns of abundance emerge from examination in the illustration given below: there is a clear increase from north to south, and abundance tends to be higher over the continental shelf (Pillar et al., 1992), or at the region of the shelf break (Barange, 1994; Gibbons, 1995) than it is in the open ocean. Such patterns in abundance and biomass generally reflect patterns in environmental productivity such as coastal and shelf-edge upwelling. However, high variablity and patchiness caused by local physical conditions, such as embayments (Cornew et al., 1992) and currents (Barange and Stuart, 1991), often mask obvious seasonal relationships between biomass or abundance and productivity on a large scale basis. This is compounded by the relatively long life-span of euphausiids as well as their omnivorous diet, which buffer the population against changes in the environment (Approximate distribution).
Data in the illustration given above indicate normal and occassional ranges; thus 0-10-100 indicates that euphausiids are normally encountered in the range 0-10 ind. mö-2 but sometimes occur at up to 100 ind. mö-2. The data used in the construction of this illustration have come from sources other than those given in Tables 1a and 1b: Eu table 1a, Eu table 1b,Extended legend tables 1a, 1b[/textfile][/l].
Given the diverse manner in which [l][m]Glossary[/m][r]abundance[/r]abundance data are presented in the literature, it is stressed that these data should be regarded as approximate. It should be noted too that different species of euphausiid in the same community may display different patterns of seasonality. For example, the cold-water Euphausia vallentini dominates krill communities over the shelf of southern Argentina during winter but is replaced by the smaller, "warm-water" species Euphausia lucens and Thysanoessa gregaria later in the year (Tarling, 1995). Such seasonal changes in biomass composition may reflect physical changes in circulation as much as they do physiological responses by the different species to changes in the environment.
Although there is good agreement between the data presented in the illustration above and the biogeochemical provinces of Longhurst (1995), there is greater congruence with his broad model domains. Having said that, there are areas of lesser agreement. These include the South Atlantic Tropical Gyre (SATL), and the Western (WTRA) and Eastern Tropical Atlantic (ETRA), which fall under Longhursts’ westerlie, trade and coastal domains respectively. However, such discrepencies are probably due to the paucity of the data used here.
There is, at best, a poor agreement between zooplankton production and phytoplankton biomass and production in coastal upwelling areas (Hutchings, 1992; Hutchings et al., 1995). This is a product of differences in population response times and results in much of the phytoplankton entering sedimentary pathways. Nevertheless, euphausiids reach their highest abundances in the South Atlantic in the coastal upwelling regions off Africa (BENG), and as noted previously, this must partly reflect an omnivorous diet. Although zooplankton and phytoplankton may be more closely coupled in the oceanic and tropical areas (SATL, WTRA and ETRA), low euphausiid biomass there may be exaggerated as a result of a greater emphasis on carnivory than in coastal waters.