Because different communities of organisms are found in different parts of the sea, it is possible to distinguish particular bodies of water not only by their physical and chemical features, but also to some extent by their characteristic populations. Moving water carries with it an assortment of planktonts which can be regarded as natural drift bottles, and by observing the distribution and intermingling of different planktonic populations it may sometimes be possible to trace the movement and mixing of the water.
Large easily identified planktonts which are characteristic of particular bodies of water serve as convenient labels or 'indicators' of the water. Pelagic larvae of certain benthic species sometimes have special value because when they appear in an area where the adults do not occur, or if present do not breed, these larvae must have been carried there by the flow of water. The study of the distribution of plankton indicators has several advantages as a method of investigating water movements. It may be simpler to obtain plankton samples than hydrographic data; and the biological evidence may be more informative because, where mixing occurs between different bodies of water, their characteristic populations may remain recognizable longer than any distinctive hydrographic features can be detected. Where conditions are changing, some species quickly succumb while hardier organisms survive longer. The distribution of a range of species of different tolerances may therefore give some indication of alterations in the quality of water as it moves from place to place. Plankton indicators are not as important as they used to be and their use has now largely been superseded by modern detection methods including the use of anthropogenic tracers (see page 51). However, they can still signify changes which may have been missed by current meters and other hydrographic indicators and it is still interesting to look at some of the classic work done in this field. As an example, we will consider the distribution of some planktonic species around the British Isles.
During the period 1920-1927, Meek (1928) studied the distribution in the northern part of the North Sea of the two commonest chaetognaths of the area, Sagitta setosa and Sagitta elegans, and correlated his findings with hydrographic data. He observed that the two species are seldom found together. S. setosa occurs throughout the greater part of the North Sea, and is the only planktonic chaetognath normally found in the southern part of the North Sea. Inshore along the east Scottish coastline S. setosa is less common, and here S. elegans usually predominates. Further south along the Northumbrian coast, the abundance of the two species fluctuates. Meek concluded that when hydrographic conditions indicate a strong flow of Atlantic water into the northern part of the North Sea, S. elegans spreads further south, mainly in the inshore waters along the British coast. Alternatively, when the flow of Atlantic water into the North Sea is weak, the distribution of S. elegans retreats and S. setosa extends further north.
S. elegans and S. setosa both occur in the English Channel, and their distribution there was investigated by Russell (1935, 1939). As in the North Sea, the two species have rather different distributions which vary from time to time. Prior to 1931, S. elegans was dominant to the west of Plymouth, S. setosa to the east. In the 'elegans' water Russell found several other planktonic animals often present, including the medusae of Cosmetira pilosella, the hydroid stage of which does not usually occur in the Channel but is found in deeper water along the Atlantic shores of the British Isles, also the trachymedusan Aglantha digitale, the pteropod Clione limacina and the euphausids Meganyctiphanes norvegica and Thysanoessa inermis. These were absent from the 'setosa' water further east. This group of organisms found in 'elegans' water was thought to come from the west, flowing into the Channel from the Celtic Sea area to the south of Ireland and around the Scillies. Russell designated this water as 'western' water to distinguish it from the water of the main part of the English Channel, and also from 'south-western' water which occasionally flows into the Channel from the Bay of Biscay bringing a different characteristic collection of warmer-water forms, notably the copepod Euchaeta hebes and the medusa Liriope tetraphylla. It is now known that the species associated by Russell with 'western' water usually enter the Channel from the north-west rather than from the west, and consequently they are nowadays referred to as 'north-western' forms.
After 1930, the boundary between Channel water and 'north-western' water, as indicated by the distribution of the two species of chaetognath, lay further west than previously, approximately at the longitude of Land's End. This shift of the boundary was accompanied by changes in the quality of the water off Plymouth. Mean water temperature increased by about 0.4°C compared with the previous thirty years. Chemically, a lower concentration of phosphate during the winter months was observed. Biologically, it was noted that in subsequent years there were changes in the population, the plankton becoming sparser and less varied with a poorer survival of most types of fish larvae during the summer months. Herring shoals became so much reduced that the Plymouth fishery was eventually abandoned. The changed conditions, however, seem to have favoured the pilchard, which increased in numbers in the Channel as the herring declined.
Work by Russell (1973) and Southward (1974) showed that after 1965 there was a return to the pre-1930 conditions in the English Channel. S. elegans occurred in much greater numbers off Plymouth than during the 1930-60 period, the plankton was richer with much larger numbers of fish larvae surviving, and there were more herring and fewer pilchards. These changes were associated with a slight climatic shift towards cooler conditions following the exceptionally cold winter of 1962-3, favouring north-western species rather than southern (Reid, 1975). The mean sea temperature dropped by about 0.4°C and the seasons were slightly later.
Russell also related the distribution of the two species of chaetognath to the rate of flow of the current through the Strait of Dover, normally a net flow of water eastwards from the Channel into the North Sea. Russell showed that the stronger this current through the Strait, the further eastwards S. elegans spreads up the English Channel and vice versa. In both the western part of the Channel and the northern part of the North Sea, S. elegans and S. setosa are regarded as indicators of different qualities of water. Studies of the overall distribution of the two species have shown that, around the British Isles, S. setosa is restricted to the neritic water of lowish salinity found in the Channel, the North Sea, the Bristol Channel and Irish Sea. S. elegans is also a neritic form but occurs in slightly more saline water than S. setosa. It predominates in those regions where oceanic and coastal water become mixed, and is consequently limited around the British Isles chiefly to the western and northern areas and the northern part of the North Sea. One factor influencing its distribution is the extent to which Atlantic water flows over the continental shelf. The boundary between the two species is by no means distinct. In the western English Channel they sometimes exist together, one or the other being more numerous at different times and places, and it is often difficult to know to what extent these variations are due to either circulatory or climatic changes.
Following the early investigations by Meek and Russell, plankton surveys around the British Isles have provided further information about the distribution of organisms which can be related to particular areas of water (Lee and Ramster, 1977; Colebrook et al., 1991). It is beyond the scope of this book to attempt any detailed account of this complicated subject but the following is a brief summary. First we should note that the water of the North Sea and English Channel is derived from several sources, being a mixture of ocean water entering through the western mouth of the Channel and the northern mouth of the North Sea, and diluted by appreciable quantities of low-salinity water from the Baltic and fresh water from rivers (Lee, 1970). The plankton of the area therefore includes endemic neritic forms, to which may sometimes be added oceanic species carried in from deep water, and estuarine or brackish species carried out to sea.
The plankton of the eastern end of the English Channel, the southern part of the North Sea and much of the Irish Sea consists predominantly of organisms which prefer a slightly reduced salinity and are appreciably euryhaline and eurythermal to suit the somewhat fluctuating conditions of the region. This water is indicated by the presence of S. setosa. The copepods Temora longicornis, Centropages hamatus, Isias clavipes, Labidocera wollastoni and Oithona nana are common in the area, and the phytoplankton often includes Biddulphia sinensis, Asteroniella japonica, Nitzchia closterium, Eucampia zodiacus and Bellarochia malleus. Because this water is shallow and close to extensive coastlines its plankton also contains many meroplanktonic forms, eggs, spores, medusae and larval stages of innumerable benthic and intertidal organisms which are not often found in more remote, deeper ocean water. These constituents vary seasonally according to the breeding habits of the different contributors and are a characteristic feature of neritic plankton.
In the northern part of the North Sea, the western part of the English Channel and off the west and north coasts of the British Isles, there is a higher proportion of water of oceanic origin than occurs in the southern North Sea and eastern Channel. The higher concentration of nutrients derived from deep water gives a rather richer and more varied plankton than that of 'setosa' water. We have already referred to this area of mixed oceanic-neritic water in connection with the distribution of S. elegans, and other organisms found in this water include the trachymedusa Aglantha digitale, the copepods Metridia lucens, Candacia armata and Centropages typicus, the euphausids Meganyctiphanes nervegica, Nyc-tiphanes couchi, Thyanoessa inermis and T. longicaudata, the polychaete Tomopteris helgolandicus and the pteropod Spiratella retroversa. All these animals are not confined to the same water as S. elegans. Most of them are also abundant in deep water further west and north. T. inermis and N. couchi are essentially shallow-water forms approximately limited to 'elegans' water. Another euphausid, T. raschii, occurs in 'elegans' water and also extends right across the northern part of the North Sea to the Skagerrak. The distribution of several species often alters seasonally in a somewhat irregular way. Along the east coast of Scotland and England there is a general tendency for the 'elegans' association to spread southwards during summer and autumn towards the southern part of the North Sea. But T. inermis does not usually penetrate far south in the North Sea, where it is typically found only in spring in the northern part. N. couchi is usually absent from the North Sea in summer but appears in the north in autumn and spreads south during winter. The population of 'elegans' water also contains many meroplanktonts from the shore and sea bottom, which change seasonally. As in the English Channel, the 'elegans' water of the North Sea has larger numbers of surviving fish larvae than the 'setosa' water.
Ocean water flowing over the continental shelf towards the British Isles comes from three main sources: (a) the North Atlantic, (b) the Arctic and (c) the Bay of Biscay and Mediterranean (Figure 4.15):
(a) The major influence is the North Atlantic Drift, driven north-eastwards from lower latitudes by the westerly winds and bringing with it a population of oceanic species. These may reach the western end of the Channel and Irish Sea, or, when the flow of North Atlantic Drift water is strong, may be carried right round the north of Scotland and enter the northern part of the North Sea. The water derives from a vast expanse of ocean, and the population it carries varies somewhat with the direction of inflow. Distinctive species from the west of Ireland include the chaetognath Sagitta tasmanica, the copepods Rhincalanus nasutus, Pleuromamma robusta and Mecynocera clausi, and the euphausids Nematoscelis megalops and Euphausia krohnii. An inflow of water from further south (south-western water) may bring with it the chaetognath S. serratodentata (atlantica), the trachymedusan Liriope tetraphylla, the siphonophoran Muggiaea kochi, the copepods Euchaeta hebes and Centropages bradyi and the pteropod Spiratella leseuri. Depending upon movements of the water, all these Atlantic forms may become very intermingled and are sometimes carried far to the north. In the northern hemisphere, all salps, doliolids, heteropods and species of Pyrosoma are said by Russell to be of warm-water origin, but Ihlea asymmetrica and Salpa fusiformis are carried well to the north of the British Isles.
(b) If water from the Arctic spreads southwards around the British Isles, it may bring with it a cold-water population including the chaetognaths S. maxima and Eukrohnia hamata, the copepods Calanus hyperboreus and Metridia longa, and the pteropod Spiratella helicina. Being cold, this water tends to move under the surface layers.
(c) Water of Mediterranean origin reaching the British Isles derives from the bottom outflow through the Strait of Gibraltar. Most of this water joins the Atlantic deep current, but a stream appears to diverge northwards and sometimes upwells along the continental slope to the north-west of the British Isles. It has been named 'Gulf of Gibraltar' water and carries a Lusitanian plankton which may include the chaetognath S. lyra, species of Sapphirina, the scyphomedusan Pelagia noctiluca and various siphonophorans, salps and doliolids.
There are, of course, many common species which are ubiquitous throughout the north-east Atlantic, and therefore not indicative of any particular body of water in the area. Examples are Calanus helgolandicus, Acartia clausi, Pseudocalanus elongatus and Paracalanus parvus. Also, many of the organisms mentioned here as indicators have an extensive distribution beyond this area. For instance Centropages hamatus and Temora longicornis, both listed above as restricted mainly to the water of slightly lowered salinity in the North Sea, eastern English Channel and Irish Sea, occur also on the western side of the Atlantic in neritic water off Nova Scotia and Newfoundland. However, neither of them is normally to be found in the intervening deep water far from the continental shelf.
Much caution is necessary in interpreting plankton indicator studies. The distribution of any planktont changes not only with movements of water but also with variations of temperature or other parameters. Consequently the gradual extension of a particular plankton population into an adjacent area may be due less to an actual inflow of water than to some alteration in water quality associated with climatic or biological change, favouring one group of species at the expense of another. So far as attempts have been made to correlate direct current measurements with observed changes in distribution of plankton indicators, these have not been invariably successful. Changes in boundaries between different plankton populations are certainly not caused solely by water movements. Nevertheless, when planktonts sporadically appear outside their normal range, it is very likely that they have been carried there by the water.
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