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Seagrass ecosystems exist throughout Pacific Island Countries and Territories (PICTs). Despite this area covering nearly 8% of the global ocean, information on seagrass distribution, biogeography, and status remains largely absent from the scientific literature. We confirm 16 seagrass species occur across 17 of the 22 PICTs with the highest number in Melanesia, followed by Micronesia and Polynesia respectively. The greatest diversity of seagrass occurs in Papua New Guinea (13 species), and attenuates eastward across the Pacific to two species in French Polynesia. We conservatively estimate seagrass extent to be 1446.2 km2, with the greatest extent (84%) in Melanesia. We find seagrass condition in 65% of PICTs increasing or displaying no discernible trend since records began. Marine conservation across the region overwhelmingly focuses on coral reefs, with seagrass ecosystems marginalised in conservation legislation and policy. Traditional knowledge is playing a greater role in managing local seagrass resources and these approaches are having greater success than contemporary conservation approaches. In a world where the future of seagrass ecosystems is looking progressively dire, the Pacific Islands appears as a global bright spot, where pressures remain relatively low and seagrass more resilient.

Five seagrass species and one subspecies distributed in two families (Hydrocharitaceae and Cymodoceaceae) are reported from the Fiji Islands and the Samoan Archipelago. The Fijian seagrass flora comprises Halophila decipiens, H. ovalis, H. ovalis subspecies bullosa, Halodule pinifolia, Halodule uninervis, and Syringodium isoetifolium. Halophila ovalis, H. ovalis ssp. bullosa, and S. isoetifolium are the only taxa recorded from Samoa. Records of Thalassia hemprichii, Cymodocea serrulata, Halophila minor, and Halophila ovata credited to Fiji and Samoa are either erroneous or remain to be verified. All the species reported herein have an Indo‐Pacific distribution, except H. ovalis ssp. bullosa endemic to Fiji, Tonga, and Samoa. Halophila decipiens is confirmed for the first time from Fiji with all specimens collected subtidally from 10–25 m depth.

The eukaryotic alga Ostreobium quekettii (Ostreobiaceae: Chlorophyceae) is reported from Fiji for the first time. It is found creeping over the calcareous plates of dead specimens of species of a barnacle genus Acasta (Pendunculata: Acastinae), which was endozoic in the gorgonian Rumphella suffruticosa. The barnacles are lodged in cavities in the coenenchyme of the gorgonian, mostly in the vicinity of colonial branching points. There is little external evidence of the location of the barnacles where the gorgonian tissue is thick, but where it is thin, prominent galls are visible. There is no evidence that the barnacle or algal association causes stress to the host, which was otherwise healthy in appearance. The presence of the algae would appear to be associated with the occurrence of the barnacles.

The rhodophyte Compsopogon coeruleus (Balbis ex C. Agardh) Montagne is newly recorded from Fiji, South Pacific. Plants occur in dense masses in the Wainivesi River in the province of Tailevu. Local villagers collect the plants and use them to cook a “pudding” with tinned fish, very similar to “puddings” prepared by coastal Fijians using marine species such as Gracilaria and Hypnea. This is the first documented account of the use of Compsopogon as a gelling agent in the preparation of human food.

Amansia paloloensis sp. nov. (Rhodomelaceae, Rhodophyta) is described from Palolo Deep National Marine Reserve, Samoa, South Pacific. Like other members of the genus, specimens of A. paloloensis lack pseudoperiaxial cells. Only three species have hitherto been included in Amansia. Amansia paloloensis is clearly distinguished from A. loriformis R.E. Norris by its larger size, highly serrated margins, and laxly arranged rosettes of adventitious branches. Amansia multifida Lamouroux has pinnate branching, which distinguishes it from all other Amansia species, including the Samoan plants. Amansia rhodantha (Harvey) J. Agardh is clearly separated from the Samoan plants by its possession of highly corticated stems and by its tight rosettes of adventitious branches, which are morphologically distinct from the main axes. This record of Amansia is the first authenticated report of this genus sensu stricto from the South Pacific Ocean.

Aspects of transboundary, water shortage, and fisheries issues are discussed in the context of the recently completed Global International Waters Assessment (GIWA) for the Pacific Islands. The region encompasses some 30 million km2 and approximately 12% of the world's ocean space, and features great geographic, demographic and developmental diversity. Global change, especially sea level rise and sea surface temperature increases, is the dominant transboundary concern as it impacts all aspects of life. Water shortage and unsustainable fishing issues are selected for discussion, as they will dominate the region into the foreseeable future, and they are illustrated with examples from Fiji, Kiribati, and Tonga. The environmental impacts are exacerbated by socioeconomic issues such as high population growth rates, urban drift, the breakdown of traditional life styles and the rapid adoption of the cash economy. Policy options that may assist in addressing these issues are proposed.

A catalogue of the marine benthic macroalgae of Fiji is provided. All records are fully referenced, and type localities and local distributions (accompanied by listings of voucher specimens) are provided. Taxonomic notes are also included where appropriate. Records from SUVA-A, UC and BISH examined by the authors, together with updates from recent publications, form the substance of the report. A total of 448 taxa are included, comprising 136 Chlorophyta, 46 Phaeophyta and 266 Rhodophyta. This represents an increase of 116 new records since 1996, of which 39 are newly reported by the authors, and the remainder are recently published records from the Suva Lagoon (29) and the Great Astrolabe Reef and Lagoon (48). Also included are a total of 36 nomenclatural changes made since 1996, a compilation of 33 uncertain records, and a list of 34 species with type localities in Fiji, Samoa and Tonga. Apart from the far more extensive region of Micronesia, the flora of Fiji is the richest in the south and central Pacific Islands.

Twenty-three taxa of Caulerpa Lamouroux are listed from the Fiji Islands, including Caulerpa reniformis sp. nov. and four taxa newly recorded [Caulerpa biserrulata Sonder, Caulerpa nummularia Harvey ex J.Agardh, Caulerpa racemosa var. lamourouxii (Turner) Weber-van Bosse and Caulerpa webbiana f. disticha Vickers]. A further four taxa are listed as Species inquirendae [C. crassifolia (C.Agardh) J.Agardh, C. juniperoides J.Agardh. C. mexicana var. pluriseriata W.R.Taylor and C. remotifolia Sonder]. A revised key to the species of Caulerpa from the Fiji Islands is presented. Verification of records is based on the personal collections of the authors and specimens housed in the Phycological Herbarium, South Pacific Regional Herbarium (SUVA-A), the University of British Columbia (UBC), the Bernice P. Bishop Museum (BISH) and the University of California at Berkeley (UC).

A total of 124 species of benthic marine algae is reported from the Palolo Deep National Marine Reserve on the island of Upolu (Western) Samoa (13°49′S, 171°45′W). Included in the catalogue are 88 Rhodophyceae, 12 Phaeophyceae and 24 Chlorophyceae. This represents the first survey of benthic marine algae for any site in the Samoan Archipelago in the last 76 years. A total of 90 species newly recorded for the region brings the total reported species to 288 (includes Cyanophyceae not reported here). This is about 68% of the current flora from, for example, nearby Fiji. Of the 11 sites investigated, the eastern algal rim of the main Deep (0–10-m depth) is the most diverse, with 68 species. The subtidal (up to 25 m deep) is also diverse with 57 species recorded. The least diverse of all the sites is the Borrow Pit with only six species.