TEMPERATURE THRESHOLD AS A BIOGEOGRAPHIC BARRIER IN NORTHERN INDIAN OCEAN MACROALGAE 1. Tom Schils 2. and Simon C. Wilson

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1 J. Phycol. 42, (2006) r 2006 by the Phycological Society of America DOI: /j x TEMPERATURE THRESHOLD AS A BIOGEOGRAPHIC BARRIER IN NORTHERN INDIAN OCEAN MACROALGAE 1 Tom Schils 2 Marine Laboratory, University of Guam, Mangilao, Guam USA and Simon C. Wilson Department of Biological Sciences, Warwick University, Coventry CV4 7AL, UK The most eastern point of the Arabian Peninsula, Ras Al Hadd, marks the boundary between the Arabian Sea and the Gulf of Oman. This geographic landmark coincides with an abrupt floristic turnover, probably one of the sharpest biotic transitions known in marine biogeography. The floras of different Arabian localities across this floristic break were compared using macrophyte distribution data throughout the Indian Ocean and seasonal seasurface temperature (SST) data. The localities from the Arabian Gulf and Gulf of Oman differ significantly from those of the Arabian Sea based on their species richness, species composition, average distribution range per species, general temperature affinity of the composing species, and seasonal temperature data of the coastal waters. Pooling the temperature data into two groups (SST 3avg, average SST of the three warmest seasons; SST min, minimum of the seasonal SSTs) revealed a temperature limit at 281 C using both the temperature affinity data of the floras and the seasonal temperatures recorded for the specific Arabian localities, which significantly separates the Arabian Sea from localities of both Gulfs. Finally, SST data of the Indian Ocean were analyzed using this upper temperature threshold of macrophytes at 281 C and the lower temperature limit of corals at 251 C, revealing general macrophyte diversity patterns. Key index words: Arabian seas; biogeography; biotic turnover; corals; Indian ocean; macroalgae; seagrass; sea-surface temperature; temperature limit; thermogeography Abbreviations: SST, sea-surface temperature; SST 3avg, average SST of the three warmest seasons; SST avg, average yearly SST; SST fd 3avg, floristically derived SST of the three warmest seasons; SST fd, floristically derived SST; SST max, maximum of the seasonal SSTs; SST min, minimum of the seasonal SSTs; SST range, yearly fluctuation in seasonal 1 Received 1 November Accepted 25 April Author for correspondence: tom@schils.be. SSTs (i.e. SST max SST min ); TSI, Tripartite Similarity Index The Arabian region (previously known as the Arabian Seas, Sheppard et al. 1992) comprises the Arabian Sea and the surrounding gulfs and seas (Red Sea, Gulf of Aden, Gulf of Oman, and the Arabian/Persian Gulf, the latter simply called the Gulf in this paper; Fig. 1) and includes some of the most extreme coastal environments of the Indian Ocean. To the North lies the shallow Arabian Gulf that experiences the most extreme annual temperature range of any marine area in the world as well as high salinities that induce physiological stresses on the Gulf s flora and fauna. Combined with the relatively recent geological origin of this basin, the waters of the Arabian Gulf harbor an impoverished subset of organisms from the surrounding region (Coles 2003). Price (2002) also reported low species richness for the Arabian Gulf, but calculated moderate levels of endemism and a high degree of b-diversity. Diversity measures, however, greatly depend on the metrics used and the geographic, biotic, or taxonomic levels addressed (De Troch et al. 2001). The diversity and ecological balance of the Arabian Gulf s benthic communities have been impacted by severe anthropogenic disturbances during the last few decades (Riegl 1999, 2002, 2003, John and George 2003, Purkis and Riegl 2005). The Gulf of Oman is a deep basin that connects the Arabian Gulf with the Arabian Sea and the rest of the Indian Ocean. As a consequence of its intermediate position and the paucity of scientific research in this area, the Gulf of Oman has frequently been included in descriptions of either the Arabian Gulf or the Arabian Sea (Olson and Dinerstein 2002). The oceanography and climate of the Arabian Sea is dominated by the alternating Indian monsoon system that induces one of the five largest upwelling systems in the world (Burkill 1999). Upwelling starts at the end of May, peaks in July August, and winds down by the end of September or the beginning of October (Savidge et al. 1990). During these months, primary productivity increases 10-fold in response to the increase in nutrient concentration induced by the cool upwelling 749

2 750 TOM SCHILS AND SIMON C. WILSON Red Sea Jubail (Saudi Arabia) Gulf of Aden Arabian/Persian Gulf Hormozgan (Iran) Gulf of Oman Dhofar (Oman) Socotra (Yemen) Muscat (Oman) Sur (Oman) RAS AL HADD Asylah (Oman) Masirah (Oman) Arabian Sea 500 km FIG. 1. Map of the Arabian Peninsula, showing the position of Arabian sites and Ras Al Hadd. (Brock and McClain 1992, Smith 1995, Burkill 1999), which stimulates a strong succession of grazers and higher trophic level organisms. As it is not directly affected by the SW monsoon winds, the Gulf of Oman remains calm over summer months. Its surface sea water temperature also rises, but is moderated by the indirect influence of the upwelling to the south, so compared with the Arabian Sea average seasonal temperature differences are less pronounced in the Gulf of Oman, and less extreme than the Arabian Gulf (Fig. 2). Hence, seasonal changes in temperature are prominent throughout the region. The most eastern point of Arabia, Ras Al Hadd ( Ras meaning headland, and Al Hadd being the Edge or Margin in Arabic; Fig. 1) marks the transition from the Gulf of Oman to the Arabian Sea and is an important landmark for both human navigators and migrating marine animals. Relatively little is known about the interaction between these two very different bodies of water (Sheppard et al. 1992, Wilson 2000) but new Temperature ( C) Musandam (Arabian Gulf) Qalhat 15 (Gulf of Oman) Dhofar (Arabian Sea) Apr May Jun Jul Aug Sep Oct Nov Dec FIG. 2. In situ seawater temperature data recorded at three sites in 2001: Musandam in the Arabian Gulf, Qalhat in the Gulf of Oman, and Hadbin in Dhofar on the Arabian Sea coast. Temperature differences during the summer months are due to the influence of the Arabian Sea upwelling, which is the strongest in Dhofar, intermediate in the Gulf of Oman, and absent in the Gulf. information about their effect on the distribution of marine life is emerging. This paper characterizes macrophyte distributions along the southern and eastern shores of the Arabian Peninsula using species richness and floristic characteristics to identify areas of high species turnover. Parameters regarding the distributional spread and temperature affinity of each Arabian macrophyte have been determined, based on sea-surface temperature (SST) images and species distribution ranges in the Indian Ocean. Relationships between species turnover, biogeographical spread, and temperature affinity have been analyzed for the different Arabian localities and their macrophyte communities. The strong correlation between a specific temperature parameter and changes in general floristic community types for the Arabian Seas have been extrapolated and tested for the wider Indian Ocean. We also describe a new method for exploring the role of temperature thresholds in explaining biogeographic patterns in marine taxa. MATERIALS AND METHODS Floristics. The distribution data of Indian Ocean macrophytes were computed in PhycoBase (Schils et al. 2004, Schils 2006), a Microsoft Access data base. PhycoBase includes various specimen and distribution data tables for Indian Ocean macroalgae and seagrasses. The latter data set is primarily based on Silva et al. (1996) and supplemented with records from omitted and recent sources: Dickie (1888), Holmes (1903), Cordero (1993), Nizamuddin and Campbell (1995), Shaikh and Shameel (1995), De Clerck and Coppejans (1996, 1999), Hayee-Memon and Shameel (1996), Jupp et al. (1996), Sohrabi Pour [sic] and Rabii (1996), Critchley et al. (1997), Spalding et al. (1998), Wynne (1998, 1999a, b, 2000, 2001a, b, 2002a, b, 2003a, b, c, 2004, 2005), Wynne and Jupp (1998), Aliya and Shameel (1999), OSP 1999, Shameel (1999), Coppejans et al. (2000, 2002, 2004), Huisman (2000), Kemp (1998a), Sohrabipour and Rabii (1999), Shameel et al. (2000), Bandeira et al. (2001), Wynne and Leliaert (2001), De Clerck et al. (2002, 2004, 2005), Huisman and Schils (2002), Schils and Coppejans (2002), Shameel (2002), Wynne and de Jong (2002), Carvalho and Bandeira (2003), De Clerck (2003), Green and Short (2003), John and George (2003), Richards and Wynne (2003) Schils et al. (2003a, b, 2004), Schils and Coppejans (2003a, b), Huisman et al. (2004), Leliaert (2004), Wynne and Fresh-water (2004), and Guiry and Nic Dhonncha (2005). The above references encompass practically all macrophyte species records for the Indian Ocean published to date. The large majority of these publications have been produced by experienced taxonomists and are to some extent validated by Silva et al. (1996). The data set is also supplemented with records from gray literature and recent field collections held at the herbarium in Ghent. The data set contains a total of 17,654 marine macrophyte records from the Indian Ocean, of which 9184 are from the Arabian Seas area. Cyanobacteria records which generally suffer from inadequate taxonomic determination were omitted from the analyses. The distribution data of Arabian macrophytes are largely based on typical sites (relevé) and transect data in which the functional groups of epiphytes and crustose algae received less attention, and so these groups have also been excluded from the analysis on a species-by-species evaluation. Furthermore, ambiguous species that occur in similar habitats were lumped in species complexes by passing all data through a taxonomic

3 THERMOGEOGRAPHY IN THE INDIAN OCEAN 751 Spp. no and Distribution range (100 km) RAS AL HADD 0 S 0 D M A S M H J O HO AS SY UR US O UB C R Marine distance (km) FIG. 3. Total species richness (full circles), mutually exclusive species richness (Arabian Sea vs. Gulfs; open circles), number of endemics (Arabian Sea vs. Gulfs; crosses) for eight Arabian sites. The squares represent the average floristic distribution range throughout the Indian Ocean for a given site (in 100 km). The dashed lines connect the values of well-investigated sites (full lines) with the preliminary estimates for Asylah. Abbreviations of Arabian sites: SOC, Socotra; DHO, Dhofar; MAS, Masirah; ASY, Asylah; SUR, Sur; MUS, Muscat; HOR, Hormozgan; JUB, Jubail. filter. For the analysis of macrophyte distributions in the eastern Arabian Seas, seven sites were selected as these represent the best-surveyed localities in the region with respect to macrophyte diversity. Because of its proximity to Ras Al Hadd, we also included an additional site (Asylah) in the species richness calculations (Fig. 3). Rough sea conditions on the exposed shores of Asylah during surveying did not allow a complete sampling of this area, and consequently data from this site have been excluded from the ordination and statistical analyses. The resulting data set for the eastern Arabian Seas consists of 354 species, 347 macroalgae (194 Rhodophyta, 89 Chlorophyta, 63 Phaeophyceae, and one Xanthophyta), and seven seagrasses (for complete record, see Supplementary Table S1). SSTs. Seasonal SSTs images (level 3 standard mapped Aqua-MODIS images) were available for Each pixel (4 km grids) within these images represents the average seasonal temperature for that spot (considered as an approximation of seabed temperature in coastal areas). Subsequently, 18 coastal areas were demarcated using the program ILWIS 3.1 (International Institute for Aerospace Survey and Earth Sciences, Enschede, the Netherlands): South Africa East; Mozambique; Réunion; Mauritius; Tanzania; Kenya and Somalia South; Seychelles; Somalia North; Yemen South; Oman Arabian Sea; Gulf of Oman; Saudi Arabia; Iran; Pakistan; Maldives; Sri Lanka; Indonesia; and Western Australia. These areas correspond to the distribution areas (countries and islands) of the investigated macrophytes, as used in Silva et al. (1996) and PhycoBase. This also enabled the distribution range of each macrophyte species to be quantified in kilometers. The average seasonal temperature per year of all coastal areas was computed following the methodology used by NASA (oceancolor.gsfc.nasa.gov): spring, March 21 June 20; summer, June 21 September 20; autumn, September 21 December 20; winter, December 21 March 20. Thereafter, the average yearly temperature for these coastlines, as well as the minimum and maximum of the seasonal temperatures was calculated. The investigated Arabian sites displayed a clear pattern of three seasons characterized by similar warm SSTs and one season of cooler SSTs. Therefore, we also computed the average temperature of the three warmest seasons. Once these four SST values were obtained for one year, averages were calculated for 2003 and The SST parameters for will be indicated by the following acronyms (Fig. 4): average yearly temperature, SST avg ; average temperature of the three warmest seasons, SST 3avg ; minimum of the average seasonal temperatures, SST min ; maximum of the average seasonal temperatures, and SST max. The difference between SST max and SST min is an additional parameter that describes the yearly fluctuations in temperature (hereafter termed as SST range ). Subsequently, the relevant SST parameters were linked with the macrophyte distribution data in a relational data base. This resulted in a floristically derived SST (SST fd ) for each Arabian macrophyte, based on the temperature characteristics of their Indian Ocean distribution range. Next, the SST fd values of all the macrophyte species recorded for a specific site were averaged, and SST fd differences across the floristic break at Ras Al Hadd were statistically tested. Two subsets of species were selected for either the Arabian Sea or the Gulfs (unity of the Gulf of Oman and the Arabian Gulf), i.e. regional endemics and mutually exclusive species, and were subjected to similar statistics. Endemics for the Arabian Sea are species, that only occur along the Arabian Sea coasts of Oman, Pakistan, northern Somalia, and Yemen. Endemics for the Gulfs are those species that have only been recorded for the Gulf of Oman, Iran, Saudi Arabia, and the United Arabian Emirates. Mutually exclusive species only occurred in one of the above marine areas, but can be found throughout other parts of the Indian Ocean. Similarity analysis and statistics. The Tripartite Similarity Index (TSI; Tulloss 1997) was used as a qualitative index of beta diversity between the different Arabian sites. The TSI values were computed in a spreadsheet, and the resulting similarity matrix served as input data for a non-metric multidimensional scaling (MDS) ordination, using the PRIMER software package (Clarke and Gorley 2001). An MDS procedure with 100 restarts was specified in order to allow the program sufficient repetition to find the best solution. The seasonal SST data of the seven Arabian sites were compared by a cluster analysis based on Bray Curtis similarities (no transformation and standardized). Clustering was also performed with PRIMER (Clarke and Gorley 2001), using the complete linkage procedure. t-tests were computed with Statistica 6.0 (StatSoft Inc., Tulsa, OK, USA), together with Levene s and the Brown and Forsythe test for homoscedasticity. If heterogeneity of variances was observed, reciprocal or SST ( C) winter 2003 spring 2003 summer 2003 autumn 2003 winter 2004 spring 2004 summer 2004 autumn 2004 FIG. 4. Seasonal sea-surface temperatures (SSTs) for Sur in , illustrating the different SST parameters used. The full circles represent the SSTs of the three warmest seasons during a year; the open circles represent the SSTs during the coolest season. SST avg, average yearly temperature based on all SSTs (all circles included). SST 3avg, average temperature of the three warmest seasons (full circles only). SST max, maximum value of all seasonal SSTs. SST min, minimum of the seasonal SSTs. SST range, difference between SST max and SST min.

4 752 TOM SCHILS AND SIMON C. WILSON square root transformations were used to normalize the data (Zar 1996). RESULTS Floristic diversity. Marine macrophyte assemblages on either side of Ras Al Hadd were analyzed for biogeographic patterns (Fig. 1) and clearly demonstrated the exceptional change in richness between the Arabian Sea and the Gulf of Oman/Arabian Gulf with a dramatic species turnover over the 50 km that spans Ras Al Hadd (Fig. 3). Starting in the south, the high diversity of the Socotra Archipelago reflected its nature as an area of biological overlap between the East African Coast and the Arabian Sea. The unique flora of Dhofar stood out as a hotspot of endemism within the Arabian Seas (Fig. 3) owing to the number of species adapted to upwelling conditions, but occurred on a gentle gradient of decreasing diversity between Socotra and Asylah. Passing Ras Al Hadd, diversity declined below 60% of the Arabian Sea level, and remained low as the transect passes northwards into the Arabian Gulf. The assemblages of the Gulfs were almost exclusively composed of ubiquitous tropical species (see distribution range in Fig. 3), while those in the Arabian Sea contained a high proportion of endemics and species with discontinuous distributions (Schils and Coppejans 2003a). Analogous to the findings of Hommersand (1986) for the South African flora, many of the Arabian Sea macroalgae were probably relicts of a once widespread warmtemperate flora during eras of global cooling. The MDS analysis (Fig. 5) of the Arabian sites showed a similar split between the Arabian Sea (Socotra, Dhofar and Masirah) and the southern Gulf of Oman (Muscat and Sur). A gradient in species composition prevailed from the southern Gulf of Oman, over DHO MAS SUR MUS HOR JUB HOR TSI JUB SOC DHO MAS SUR MUS HOR SOC MAS MUS DHO SUR Stress: 0.01 FIG. 5. Non-metric multidimensional scaling (MDS) plot of the species composition at the seven well-investigated Arabian sites based on the Tripartite Similarity Index (inset). Abbreviations of Arabian sites: SOC, Socotra; DHO, Dhofar; MAS, Masirah; ASY, Asylah; SUR, Sur; MUS, Muscat; HOR, Hormozgan; JUB, Jubail. the Strait of Hormuz, into the Arabian Gulf. Detailed taxonomic surveys are lacking for intermediate areas like northern Oman and Fujairah (UAE), but these sites are expected to follow the general trend. The species richness within the Gulfs, however, remained low in comparison with the Arabian Sea (Fig. 1). SSTs and phytogeographic temperature limit. Acluster analysis of the seasonal SST data of the seven Arabian sites formed two site groupings, corresponding to the Arabian Sea and the Gulfs (Fig. 6). In line with the floristic findings, there was a gradient in sites noticeable from the southern Gulf of Oman toward the innerpartofthearabiangulf.basedonthesstdata and floristic findings, we focused on the drastic turnover in species richness at Ras Al Hadd. A t-test of five SST parameters (SST avg, SST 3avg, SST min, SST max, and SST range ) only revealed significant differences for the average temperature of the three warmest seasons (average SST 3avg Arabian Sea: C; average SST 3avg Gulfs: C; P<0.05) between the Arabian Sea and the Gulfs (Table 1; Fig. 7). During the period of the three warmest months, the maximum seasonal temperature recorded within the Arabian Sea is C, whereas the registered minimum seasonal temperature for the Gulfs is C. Thus, the difference in SST 3avg between both areas corresponds to a temperature limit of 281 C. Hence, SST 3avg is used to calculate the floristically derived SST parameter, SST fd 3avg. Based on the species and their floristic distribution within the Indian Ocean, SST fd 3avg values were calculated for the seven Arabian sites. This revealed a significant, although minimal, difference (0.11 C; P<0.05) in the floristically derived average temperature of the three warmest months between the Arabian Sea sites and those of the Gulfs. The SST fd 3avg values of two subsets of both macrophyte floras, endemics and mutually exclusive species, showed a larger significant difference between both marine entities. Clearly, the difference was most Similarity SOC DHO MAS SUR MUS HOR JUB FIG. 6. Cluster analysis of the seasonal sea-surface temperatures of the seven Arabian sites. Clustering was based on Bray Curtis similarities and using the complete linkage procedure. Abbreviations of Arabian sites: SOC, Socotra; DHO, Dhofar; MAS, Masirah; ASY, Asylah; SUR, Sur; MUS, Muscat; HOR, Hormozgan; JUB, Jubail.

5 THERMOGEOGRAPHY IN THE INDIAN OCEAN 753 TABLE 1. Summary of a t-test of five SST parameters: SST avg, SST 3avg, SST min, SST max, and SST range. Mean Arabian Sea Mean Gulfs t-value df P SST avg SST 3avg SST max SST min SST range The calculation of the SST values is explained in the Materials and Methods. SST, sea-surface temperature; SST avg, average yearly SST; SST 3avg, average SST of the three warmest seasons; SST max, maximum of the seasonal SSTs; SST min, minimum of the seasonal SSTs; SST range, yearly fluctuation in seasonal SSTs (i.e. SST max SST min ). pronounced for the endemics (average SST fd 3avg Arabian Sea flora: C; average SST fd 3avg Gulfs flora: C; P<0.001). Mutually exclusive species for both basins showed less difference (average SST fd 3avg Arabian Sea flora: C; average SST fd 3avg Gulfs flora: C; P<0.01), because (i) some Arabian Sea sites include upwelling sheltered localities, which increased the number of taxa with warm water affinities, and (ii) the majority of taxa from the Gulfs are adapted to an extreme temperature range. The exclusive Arabian Sea flora was on average more stenothermic than those of the Gulfs (SST fd range : 4.01 C vs C; P<0.001). This also applied to the more constricted data set of regional endemics (SST fd range 3.81 C vs C; P<0.001) and to a lesser extent for the complete data set (SST fd range 4.51 C vs C; P<0.001). Next, the 281 C temperature limit that corresponded to the floristic turnover at Ras Al Hadd was extrapolated to the entire Indian Ocean. The SST 3avg values of the macrophyte distribution areas (countries and SST 3avg ( C) Arabian Sea Gulfs Median 25%-75% Min-Max FIG. 7. Box plot of the average sea-surface temperatures during the three warmest seasons (SST 3avg ). The graph displays the median, quartiles, and extreme values of SST 3avg for the three sites of the Arabian Sea and the four sites included in the Gulfs. The temperature limit at 281 C is indicated by a dashed line. islands) were compared with the 281 C temperature limit. Coastal areas within the Indian Ocean with SST 3avg values below 281 C were also characterized by SST min values lower than 251 C. Thus, following classic definitions (Kelleher et al. 1995), the diverse floras of eastern South Africa, southern Mozambique, northern Somalia, Yemen, southern Oman, Pakistan, and Western Australia could be typified as sub-tropical to warmtemperate (Fig. 8), while those above the 281 C threshold harbor tropical marine communities. Tropical reef corals thrive best in the temperature range from 25 to 291 C (Coles 2003). Below the 251 C threshold, benthic communities consisting of octocorallians and macroalgae become more dominant. If this lower temperature limit of corals is added to the graph with the upper temperature limit of macroalgae (281 C), we can further divide the tropical areas into two sub-entities. On the one hand, coastal areas with an SST 3avg above 281 C and an SST min above 251 C harbor tropical ecosystems characterized by well-developed coral reefs (Tanzania; Kenya and southern Somalia; Maldives; Sri Lanka; Indonesia). On the other, coastal areas with an SST 3avg above 281 C and an SST min below 251 C typify stressed tropical ecosystems with a reduced biodiversity and marked seasonal changes in macroalgal productivity (Arabian Gulf and the Gulf of Oman; Fig. 8). SST ( C) RAS AL HADD Marine distance (km) FIG. 8. Synthesized sea-surface temperature (SST) seasonality for the investigated Arabian sites and Indian Ocean countries. The bold full line represents SST 3avg and the dashed line SST min (see Materials and Methods). The 281 C limit corresponding to the biotic turnover at Ras Al Hadd is indicated by a horizontal full line. Coastal areas where SST 3avg <281 C are vertically hatched (warm-temperate floras) and those where SST 3avg 4281 C are horizontally hatched (tropical or stressed sub-tropical floras). The lower temperature limit (251 C) of tropical reef corals (Coles 2003) is indicated by a horizontal dash/ dotted line. Indian Ocean countries and investigated Arabian sites are listed on the abscissa. Abbreviations of Indian Ocean countries: SAE, South Africa East; MOZ, Mozambique; TAN, Tanzania; KSS, Kenya and Somalia South; SON, Somalia North; PAK, Pakistan; MAL, Maldives; SRI, Sri Lanka; INO, Indonesia; AUW, Australia West. Abbreviations of Arabian sites: SOC, Socotra; DHO, Dhofar; MAS, Masirah; ASY, Asylah; SUR, Sur; MUS, Muscat; HOR, Hormozgan; JUB, Jubail.

6 754 TOM SCHILS AND SIMON C. WILSON DISCUSSION The best-known examples of marine biogeographical transitions along the South African east coast (Bolton et al. 2004), in southern Australia (Bolton 1996), in Florida (Humm 1969), and California (Thom 1980, Murray and Littler 1981), are all characterized by overlap areas of several hundreds of kilometers while the floristic break observed at Ras Al Hadd occurs over about only 50 km. Hence, this geographic landmark probably demarcates one of the sharpest biotic transition zones known in marine biogeography. The temperature limit presented here is derived from the biotic change observed at Ras Al Hadd, and can be generalized to the larger Indian Ocean. The observed phytogeographic patterns might be extrapolated to other marine organisms, once sufficiently detailed distribution and taxonomic data become available. Previous approaches linking temperature patterns and macrophyte floras were usually confined to subsets of species (e.g. Adey and Steneck 2001) and/ or were based on generalized or annual isotherm maps (van den Hoek et al. 1990). This study is based on the spatial detail of SST data derived from satellite images. Seasonal level 3 standard mapped Aqua-MODIS images returned detailed SST values for the specific localities of interest (macrophyte sampling sites). The latter images are obtained by standard processing of the level-3 daily binned files. As we traced the coastal areas using 4 km grids for the Arabian localities and the Indian Ocean distribution ranges of macrophytes (hereby excluding cloud cover), our efforts were limited to the seasonal data that proved useful in identifying significant thermogeographic patterns. A limited number of publications (Sheppard and Salm 1988, Randall and Hoover 1995, Kemp 1998b) already indicated that the Arabian Sea sensu stricto encompasses certain centers of endemism. Such biogeographical studies in the Arabian region have always focused on patterns at a local scale, but large-scale comparative analyses are generally lacking. This study relies on detailed surveys of macroalgal assemblages along a transect from the East African Coast to the Arabian Gulf. The results show that the Arabian Sea is floristically very distinct from the Gulf of Oman and the Arabian Gulf. The biogeographical isolation of the Arabian Sea is thought to be caused by the influence of upwelling: the so-called pseudo-high latitude effect (Sheppard et al. 1992). However, our analyses failed to correlate changes in macrophyte richness with yearly or seasonal average SSTs. Annual SST minima are in fact lower in the Arabian Gulf (Jubail, Saudi Arabia) than in the Arabian Sea, and temperature fluctuations are also more extreme in the Arabian Gulf. In contrast to the stress of minimum SSTs, the average SSTs of the warmest three seasons of a year revealed a temperature limit at 281 C that corresponds to general floristic types. Thus, it appears that (sub)tropical macrophytes can withstand a season of low temperatures through a decrease in physiological activity (dormant stages, life cycle stages, aestivation, etc.). These findings are based on an extensive floristic data set including records from a wide variety of habitats and environmental conditions, combined with detailed seawater temperature data. The conclusion that the effect of a temperature threshold results in such major differences for the marine flora over the entire Indian Ocean shows that a single factor can dominate the effects of other interacting and complex environmental factors. Temperature-wise, macroalgal growth and diversity is generally drastically inhibited by upper temperature limits, whereas coral development and diversity is strongly affected by low temperature thresholds (Coles 2003). The contrast between the temperature requirements for different taxa suggests that establishing similar temperature limit/range for other benthic groups would be desirable. Analysis of combined temperature limits of key benthic groups may provide greater insight into global patterns of benthic diversity. As the latest review on eco-regions of the world (Olson and Dinerstein 2002) fails to identify important marine regions, a more integrated investigation of temperature thresholds may be useful in delimiting marine eco-regions effectively. Applications on smaller geographical scales may also be applied to marine protected areas to allow greater diversity of environmental conditions and environmental variability to be included in the selection process. Thanks are due to F. Leliaert, O. De Clerck, H. Verbruggen, and E. Coppejans for their taxonomic advice and skill. The joyful assistance of K. Pauly, B. P. Jupp, M. Zeadan Jaboob, W. Bost, A. Amer Al Kiyumi, M. Claereboudt, R. Baldwin, E. Demeulenaere, T. Collins, and P. Provoost was indispensable during the fieldwork. C. Vlaeminck kindly assisted with the data entry. T. Vanderstraete and R. Goossens are gratefully acknowledged for their support in obtaining SST data. Additionally, we appreciate the valuable input of two anonymous reviewers. T. Schils is indebted to the Fund for Scientific Research Flanders (FWO, Belgium) for his postdoctoral research grant. Financial support was provided by the FWO Research Project 3G This is contribution number 588 for the University of Guam Marine Laboratory. Adey, W. H. & Steneck, R. S Thermogeography over time creates biogeographic regions: a temperature/space/time-integrated model and an abundance weighted test for benthic marine algae. J. 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Species recorded for the 8 Arabian sites: SOC, Socotra Archipelago (Yemen); DHO, Dhofar (Oman); MAS, Masirah Island (Oman); ASY, Asylah (Oman); SUR, Sur (Oman); MUS, Muscat Area (Oman); HOR, Hormozgan (Iran); JUB, Jubail (Saudi Arabia).