Temperature tolerances and biogeography

Water temperature exerts a major control over the distribution and activities of marine organisms (Kinne, 1963). Temperature tolerances differ widely between species, but each is restricted in distribution within its particular temperature range. Some species can only withstand a very small variation of temperature, and are described as stenothermal. Eurythermal species are those of wide temperature tolerance. Strict stenotherms are chiefly oceanic forms, and their distribution may alter seasonally with changes of water temperature. Eurytherms are typical of the more fluctuating conditions of shallow water. Sessile organisms have generally a rather wider temperature tolerance than free-living creatures of the same region.

Because water temperature has so great an effect on distribution, the extent of marine biogeographical regions can be related more closely to the course of the isotherms than to any other factor. The definition of biogeographic subdivisions of the sea is inevitably somewhat vague because the marine environment contains few firm ecological boundaries. Land barriers account for some differences between oceanic populations, and wide expanses of deep water prevent the spread of some littoral and neritic species; but for the most part the transition between one fauna and another is gradual, with a broad overlap of populations. However,

Neritic Depth
Figure 4.1 Temperature profiles in the deep ocean.

in a general way the populations of the surface waters fall into three main groups associated with differences of water temperature: namely, the warm-water populations, the cold-water populations, and populations which inhabit waters of intermediate temperature where the temperature of the surface layers fluctuates seasonally, i.e. temperate waters. These major divisions of the marine population may be almost endlessly subdivided to take account of local conditions.

Warm-water populations are mainly to be found in the surface layers of the tropical belt where the surface temperature is above about 18-20oC (Figure 4.2). This warm-water zone corresponds roughly with, but is rather more extensive than, the zone of corals which have their main abundance in clear shallow water where the winter temperature does not fall below 20oC. Within the warm-water regions of the oceans there is little seasonal variation of temperature. At the Equator, the temperature of the surface water in most areas is between 26 and 27oC, and does not change appreciably throughout the year.

Cold-water populations are found in the Arctic and Southern Oceans where the surface temperature lies between about 50C and a little below 00C. In the Southern Ocean the cold water has a well-defined northern boundary at the Antarctic Convergence (see page 17) where it sinks below the warmer sub-Antarctic water. The sharp temperature gradient at this convergence effectively separates many species of plant and animal, and forms a distinct northern limit to the Antarctic faunal and floral zones. The southern boundary of the Arctic zone is less distinct except at the convergences of the Labrador Current and Gulf Stream in the Atlantic, and of the Oyo-Shiwo and Kuro-Shiwo currents in the Pacific. Broadly, the Arctic zone comprises the Arctic Ocean and those parts of the Atlantic and Pacific Oceans into which Arctic surface water spreads, the limiting temperature being a summer maximum of about 50C.

The temperate sea areas lie between the 5 and 180C mean annual surface isotherms, and here the surface water undergoes seasonal changes of temperature. The colder part of the temperate regions between the 5 and 100C isotherms are termed the Boreal zone in the Northern hemisphere and the Antiboreal zone in the Southern hemisphere.

The course of the surface isotherms is determined largely by the surface circulation. On the western sides of the oceans the warmest water reaches higher latitudes, and the coldest water lower latitudes, than on the eastern sides. The temperate zones are therefore narrow in the west and much wider in the east, where they extend further to both north and south. On the basis mainly of water temperature we can designate some of the chief biogeographic subdivisions of the littoral and epipelagic zones as follows, their positions being shown in Figure 4.2.

1 Arctic and Subarctic regions.

2 East Asian Boreal region.

Marine Biogeographic Regions

Figure 4.2 Approximate positions of mean annual isotherms and marine biogeographic areas.

Figure 4.2 Approximate positions of mean annual isotherms and marine biogeographic areas.

3 North-west American Boreal region.

4 Atlantic Boreal region.

5 North Pacific warm temperate region. East Asian province.

6 North Pacific warm temperate region. West American province.

7 Atlantic warm temperate region (Lusitanian).

8 Tropical Indo-West-Pacific region.

9 Tropical East Pacific region.

10 Tropical Atlantic region.

11 South Pacific warm temperate region.

12 South Atlantic warm temperate region.

13 Indo-Australian warm temperate region.

14 Antiboreal region.

15 Kerguelan region.

16 Antarctic and Subantarctic regions.

There are some cases of the same species, or very closely related forms, occupying zones of similar temperature in middle or high latitudes in both northern and southern hemispheres, although absent from the intervening warm-water belt. Such a pattern of distribution is termed bipolar. The bipolar distribution of a pelagic amphipod Parathemisto gaudichaudi is shown in Figure 4.3, approximating to the distribution of surface water between 5 and 10°C. Among numerous examples of bipolarity, Ekman (1953) mentions the following inhabitants of the North-East Atlantic, the barnacle, Semibalanus balanoides (North Atlantic, Tierra del Fuego and New Zealand); the tunicates Botryllus schlosseri and Didemnum albidum (both North Atlantic and New Zealand); the genus Engraulis (anchovies) and the entire order Lucernariida. In some cases apparent bipolarity is really a continuous distribution through the colder layers of water underlying the warm surface layers of the tropics, i.e. tropical submergence. Eukrohnia hamata (Alvarino, 1965) (Figure 4.4), Parathemisto abyssorum and Dimophyes arctica are examples of amphipod species found at the surface in both Arctic and Antarctic waters, and present at deeper levels at low latitudes.

Two major biogeographical provinces meet around Great Britain: the Lusitanian to the south and the boreal, which is centred on the British Isles. The British Isles lie across the 10°C mean annual surface isotherm, and in winter the 5°C isotherm moves south along these coasts. It is therefore possible here, to distinguish certain species as belonging to a northern group of Arctic and Boreal forms, and others as a southern Lusitanian group of Mediterranean and temperate water species. There are seasonal changes in distribution and a broad overlap of populations, but the 10°C isotherm lies approximately between the two groups. Among the fishes of the area, the northern group includes cod (Gadus morhua), haddock (Melanogrammus aeglefinus), ling (Molva molva), plaice (Pleuronectes platessa), halibut (Hippoglossus hippoglossus), and herring (Clupea

Mean Annual Isotherms

Figure 4.3 Approximate known distribution of Parathemisto gaudichaudi in the Atlantic, and mean annual isotherms for 5°C and 10°C.

(Based on a map of the world by courtesy of G. Philip and Son Ltd.)

Figure 4.3 Approximate known distribution of Parathemisto gaudichaudi in the Atlantic, and mean annual isotherms for 5°C and 10°C.

(Based on a map of the world by courtesy of G. Philip and Son Ltd.)

Bering Sea Depth Map

Figure 4.4 Distribution in depth of Eukrohnia hamata in the Pacific, from the Bering Sea to MacMurdo Sound.

(From Alvarino (1965) by courtesy of Allen & Unwin.)

Figure 4.4 Distribution in depth of Eukrohnia hamata in the Pacific, from the Bering Sea to MacMurdo Sound.

(From Alvarino (1965) by courtesy of Allen & Unwin.)

harengus). Examples of southern forms are pollack (Pollachius pollachius), European hake (Merluccius merluccius), Dover sole (Solea solea), turbot (Scophthalmus maximus), pilchard (Sardina pilchardus), anchovy (Engraulis encrasicolus), mackerel (Scomber scombrus) and tunny (Thunnus thynnus and T. alalunga).

On the seashore almost all boreal species can occur all round the British coast but a few (e.g. the barnacle Semibalanus balanoides, the limpet Acmaea tessulata, the blenny Zoarces viviparus and the spindle shell Neptunea antiqua), which are common in the north and east, become scarce or absent towards the south-west. There are a larger number of species which are abundant in the south-west and often extend up the west coast, but are absent in the north and east, the British Isles being the northernmost limit of their range. These southern forms include the barnacles Chthamalus montagui, C. stellatus, and Balanus perforatus, the top shells Monodonta lineata, and Gibbula umbilicalis, the limpet Patella depressa, the snakelocks anemone Anemonia sulcata, the prawn Leander serratus and the cushionstar Asterina gibbosa.

Long-term changes in mean annual sea temperatures and short-term weather extremes can alter local patterns of distribution. For example, the distribution of Monodonta lineata eastwards along the coast of the English Channel appears to be affected by events such as the unusually cold winter of 1962-3. After about 1961, mean annual sea temperatures around the British Isles fell slightly compared with those of the previous 25 years. The distribution of many marine organisms correspondingly shifted slightly southwards. In the western part of the English Channel cold-water species such as cod, Norway pout, ling and herring became more numerous, whereas warm-water species, notably pilchards and hake, declined in numbers over the same period and tended to spawn later. Effects could also be seen on the relative abundance of some species of barnacles (see page 288). Around south-western shores, numbers of the southern Chthamalus montagui and C. stellatus steadily increased, along with sea temperature, until around 1960 (although the variation in sea temperature was only about 0^C in 50 years). After 1961 or so, mean sea temperatures fell again and the relative proportion of the boreal Semibalanus balanoides started to increase. In S. balanoides, high summer temperatures prevent final maturation of gametes and may also affect adult survival. Now sea temperatures in the North Atlantic seem to be rising again and we may see a further shift in these proportions.

Over the past 15 years or so, there has been a great increase in the number of sublittoral surveys using divers, around the coastlines of the UK and Ireland (see page 231). This has resulted in many new records of the northern and southern distributional limits of species as well as new UK records (Earll and Farnham, 1983; Erwin et al., 1990; JNCC, 1995). During the summer in the UK, there are regular reports of warm-water fish species such as seahorses (Hippocampus) and trigger fish (Balistes) extending northwards into the English Channel. Sometimes these species may even overwinter here, but they cannot successfully breed. The effect of water temperature on breeding is one of the key factors influencing the distribution of marine organisms (see Section 4.2.4). The exceptionally hot summer of 1995 resulted in the appearance of many exotic species in UK waters. In Cornish waters, a big-eyed thresher shark (Alopias superciliosus) commonly found off Florida, and a sailfin dory were landed by fishing boats - the first records of these species in UK waters. Other visitors included mako and hammerhead sharks in the Irish Sea, bluefin tuna in Shetland and sunfish in the North Sea. Sea temperatures in the North Atlantic are now rising and such tropical species could become regular visitors if the warming continues. The Marine Biological Association in Plymouth (UK) has established a British Marine Fishes Database. It is currently being used as a tool for monitoring the distribution and abundance of uncommon species in response to environmental change.

Whereas the distribution of many littoral, sublittoral and epipelagic species is fairly fully recorded, knowledge of the distribution of species at deep levels is very incomplete. It is becoming apparent that there is greater diversity of abyssal species than was earlier thought, and that some are relatively restricted in distribution. However, the limits of abyssal zoogeographic regions are even less clearly defined than those of the surface layers. The most distinct deep-level boundary is probably the system of submarine ridges separating the Arctic basin from the North Atlantic which, together with the shallow water of the Bering Strait, form a barrier which some abyssal species do not cross. North of the North Atlantic Ridge much of the bottom water of the Arctic is colder than 00C, while throughout the rest of the abyss almost all the water lies between 0 and 40C. Relatively few species appear to be common to the bottom of both Arctic and other deep oceans. The deep levels of Atlantic, Indian and Pacific Oceans are all connected by the deep water of the Southern Ocean, and throughout this vast extent of abyss many species are widely distributed.

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