Stream Macroalgae of the Hawaiian Islands: A Floristic Survey

Between January 2001 and May 2003, 167 stream segments on the islands of Kaua‘i, O‘ahu, Maui, and Hawai‘i were sampled for stream macroalgae and measured for a series of physical and chemical conditions. Conditions ranged more widely than previously reported, which is likely due to the greater diversity of habitats accessed and the year-round sampling representation in this study. Water temperature ranged from 12.5 to 27.5 C (mean 1⁄4 21.4 CG 2.4), pH from 5.5 to 8.9 (mean 1⁄4 7:8G 0:5), and specific conductance from 20 to 490 mS cm 1 (mean 1⁄4 102 mS cm 1 G 75:9). A total of 160 specific and subspecific taxa was identified, of which 27 are new records for the Hawaiian Archipelago. The Chlorophyta compose the majority of the taxa, followed by the Cyanobacteria, Rhodophyta, Bacillariophyta, and Tribophyta. The mean number of taxa per stream segment was 5.0G 2.7, which is the highest such value reported. Grouping of taxa by morphological form demonstrates that the majority of taxa were free filaments (58%), followed by mats (17%), tufts (13%), and gelatinous colonies (9%). A principal coordinates analysis of the stream sites indicated that a high degree of overlap in floristic composition is evident for most of the Islands, and only sites on the island of Hawai‘i exhibit a localized positioning to one side of the principal coordinates bi-plot. The flora of Hawai‘i Island appears to be unique only in the sense that it contains fewer broadly distributed taxa than the remaining islands, which may be a function of island age. Cluster analysis of the islands based on two types of comparisons suggests stronger similarities between the islands of Maui and Kaua‘i, and O‘ahu and Hawai‘i than previously reported. The Hawaiian stream macroalgal flora contains a number of cosmopolitan taxa, although it is recognized that concepts of some of these taxa may change with additional data. Located more than 3,500 km from the nearest continental landmass, Hawai‘i is the most isolated archipelago in the world ( Juvik and Juvik 1998). This chain of islands and atolls is recognized as an outstanding outdoor evolutionary laboratory due to a combination of extreme isolation, a broad range of available habitats, and a unique biota that has evolved following colonization by waif individuals ( Juvik and Juvik 1998). Many organisms have been identified as examples of adaptive radiation in Hawai‘i, including members of the angiosperms (which, as a group, are estimated to be 89% endemic) and the insect fauna (estimated to be 99% endemic) (Wagner and Funk 1995). The marine algae of the Hawaiian Islands are estimated to be 20–25% endemic, which is a moderate value for marine systems (Hawaiian marine systems are typically characterized by 25–35% endemism [Zeigler 2002]). Hawaiian marine algae have been actively studied for many years from a taxonomic perspective, culminating in the recent publication of several monographic works on Hawaiian red, green, and brown seaweeds (Abbott 1999, Abbott and Pacific Science (2006), vol. 60, no. 2:191–205 : 2006 by University of Hawai‘i Press All rights reserved 1 This study was funded by the Hawai‘i Division of Aquatic Resources. A.R.S. was supported during part of the study by a postdoctoral fellowship from the Natural Sciences and Engineering Research Council of Canada. Manuscript accepted 2 June 2005. 2 Department of Botany, 3190 Maile Way, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96822 (e-mail: [email protected]). Huisman 2004). Algae have also played an important role in Hawaiian culture. Edible algae, limu in the Hawaiian language, have been critical to the diet of native Hawaiian peoples for many hundreds of years (Abbott 1984). The vast majority of limu species are marine; however, a few freshwater and brackish examples are known, indicating that at least a few stream algae species were also culturally important (Abbott 1984). During the 1950s to the 1980s, a number of seaweed species were introduced to Hawai‘i for production of compounds such as agar and carrageenan (Russell 1983, Rodgers and Cox 1999). Some alien species have since become naturalized in the Hawaiian flora, in several cases overwhelming the natural algal assemblages and growing to nuisance levels (e.g., Smith et al. 2002, 2004). The freshwater algae of Hawai‘i, by comparison, have received scarce attention over the past 130 yr. A bibliographic checklist of the nonmarine algae (freshwater and terrestrial) of the Hawaiian Islands indicates that 800 specific and subspecific taxa have been reported from Hawai‘i (both microalgae and macroalgae), of which a mere 5.0% are believed to be endemic (Sherwood 2004). Knowledge of the stream macroflora of Hawai‘i lags far behind that of the seaweeds, and only preliminary surveys discussing the taxonomic composition of stream algae are recorded in the published literature (e.g., Vis et al. 1994, Filkin et al. 2003, Sherwood et al. 2004). Still, these surveys highlight the great biodiversity in Hawaiian streams, as well as the potential for discovery of undescribed taxa. As an example, 25 of 34 taxa reported in the first survey of Hawaiian streams represented new records for the Archipelago, and one new species was described: the rhodophyte Batrachospermum spermatiophorum Vis & Sheath (Vis et al. 1994). Determination of the native Hawaiian stream algal flora is virtually impossible. Such information is readily available for many other organismal groups, including angiosperms (e.g., Wagner et al. 1999), fishes (e.g., Randall 1996, Yamamoto and Tagawa 2000), and marine algae (Abbott 1999, Abbott and Huisman 2004). However, because the historical record of freshwater algae in Hawai‘i is patchy at best, only an incomplete picture is available of the stream flora before the first Polynesian settlements (between AD 400 and 1100) and European contact (AD 1778) (Zeigler 2002). Although two ethnobotanical uses of brackish and freshwater algae are mentioned by Abbott (1984), the vast majority of limu are marine seaweeds, and the stream algal flora did not play nearly as important a role in the diet of native Hawaiians as the marine flora. It is almost certain that the stream algal flora was altered after human settlement of the Islands because the importation of animal species and cultivation of food crops from other Pacific islands could also have introduced freshwater and terrestrial algae species to Hawai‘i. This scenario seems especially likely for cases such as drinking water and crop plants (which may have had soil surrounding the roots), brought by early Polynesians (e.g., Zeigler 2002). Two different approaches could be taken to infer the precontact stream algal flora, but neither is completely satisfactory. First, it could be assumed that the algae currently growing in pristine areas on each island represent refuge elements of the flora that were present before human contact and influence. Second, molecular genetic data could be used to pinpoint the divergence between congeners from possible colonization sources. Given the difficulty of such determinations, and the current lack of such data, this study makes no distinctions between taxa that may have been present in Hawaiian streams before human intervention and those that have since been introduced. This study represents the most thorough survey of freshwater algae from Hawaiian streams. Algal stream surveys from other tropical as well as temperate areas over the last several decades have provided a comparative basis for this research (e.g., John and Moore 1985, Entwisle 1989, Necchi and Moreira 1995, Sheath and Cole 1996). The focus of the study reported here is on stream macroalgae; that is, those benthic algae that have a discrete thallus visible to the naked eye (Sheath and Cole 1992). Macroalgae are known to play an important ecological role 192 PACIFIC SCIENCE . April 2006