Biology Reference
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18.2 Coastal Marine Biogeographical Regions and Their
Relation to Oceanic Isotherms and Temperature
Responses of Seaweeds
Especially temperature changes had and will have strong effects in the geographical
distribution of seaweeds as temperature is the main abiotic factor directly
controlling geographic boundaries of seaweed species (van den Hoek 1982a ;
L
uning 1990 ; Chap. 3 by Eggert) and also has recently been identified in shaping
global marine biodiversity pattern (Tittensor et al. 2010 ). Principally, the presence
of seaweeds in these regions is determined by the physiological tolerance of their
life cycle stages to temperature. There are two fundamentally different types of
temperature boundaries in seaweeds, lethal boundaries and growth or reproduction
boundaries (Breeman 1988 ; van den Hoek and Breeman 1989 ). Lethal boundaries
are determined by the capacity of the species to survive during the unfavorable
season, e.g., a cold-water alga in the summer season. Growth and/or reproduction
boundaries are determined by the ability of the species to grow and reproduce
during the favorable season, e.g., a cold-water alga in the winter/spring season.
Moreover, seaweed distribution is affected by temperature-daylength interactions
(Dring 1984 ; Molenaar 1996 ) affecting the coordinated timing between life cycle
stages.
Seven broad biogeographical regions have been recognized in coastal marine
zoogeography and phytogeography (Briggs 1974 , 1995 ;L
uning 1990 ): the tropical
region of the Atlantic and Indo-Pacific, the cold- and warm-temperate regions of
both hemispheres, and the two Polar regions (for details see Chap. 3 by Eggert).
Although Spalding et al. ( 2007 ) recently proposed a new and more sophisticated
global system of marine ecoregions than Briggs ( 1995 ), the major biogeographical
boundaries along the continents are similar in both approaches. We therefore chose
the Briggs ( 1995 ) system as it is better suited for demonstrating the broad global
pattern of modeled future change. The boundaries between biogeographical regions
after Briggs ( 1995 ) are determined by fundamental changes in the composition of
coastal biota and have been defined by their degree of endemism (
10%; Spalding
et al. 2007 ). The transition between regions may be characterized by SSTs as
temperature is a major factor in shaping marine phytogeographical regions (e.g.,
van den Hoek 1982a , b ;L
>
uning ( 1990 ) described the boundaries of
phytogeographical regions by intermediate mean summer and winter isotherms to
show the general broad pattern. The specific mean summer and winter isotherms
prevailing at the eastern and western coasts of the Oceans and in the northern and
southern hemisphere at the phytogeographic boundaries deviate, however, consid-
erably from these averages and are presented in Table 18.1 based on isotherm data
from 1980 to 1999 provided by Muller et al. ( 2009 ).
The current tropical coastal biogeographical regions are broad and characterized
by summer isotherms between 24 and 29 C and winter isotherms between 18 and
23 C (Table 18.1 , Figs. 18.1a and 18.2a ). The adjacent warm-temperate regions are
much smaller and generally slightly compressed on the western sides of the Pacific
uning 1990 ). L
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