Spatial and temporal variation of diverse inter‐tidal algal assemblages in Southwest O‘ahu

Abstract

Spatial and temporal variation in tropical inter‐tidal communities is poorly known, making predictions about the effects of climate change and other anthropogenic disturbances difficult. Along Southwest O‘ahu, Hawai'i, local residents are concerned about the environmental effects of coastal development and the perceived loss of targeted algal species, which are collected for human consumption. To describe the coastal benthic community and better understand the processes that form and maintain it, the abundance and composition of macroalgae were sampled in the region's inter‐tidal zone from 2006 to 2015. Sixty‐six macroalgal species and two broad algal assemblages were identified that corresponded to substrate topography and sand influence at a similar tidal elevation. Along flat carbonate benches with a sand beach, Phaeophyceae and Rhodophytes occurred in almost equal proportions, while shores with slightly more topographic relief and angular substrate were dominated by Rhodophytes. Foliose or turf algal forms were most common. Surveys captured the local invasion of an alga, Avrainvillea sp. and significant declines in abundant macroalgae in 2015 after a period of unseasonably warm, calm water. Temporal changes in algal assemblages were related to maximum water temperature and wave height but not precipitation. Thus, algal assemblages appear to be structured by local beach morphology as they interact with sand and wave activity and episodically by unusual weather events. However, manipulation and continuous monitoring of the algal assemblages coupled to sensing of the localized environment is necessary to confirm factors related to assembly maintenance and recent species shifts.

A new record of Avrainvillea cf. erecta (Berkeley) A. Gepp & E. S. Gepp (Bryopsidales, Chlorophyta) from urbanized estuaries in the Hawaiian Islands

“…“ A. amadelpha ” now inhabits the intertidal, subtidal, and mesophotic environments in Hawai‘i (Spalding 2012), and is considered invasive (Peyton 2009, Cox et al 2017), altering the benthic ecosystem (Martinez et al 2009), and competing with native species (Peyton 2009). Given these characteristics, considerable attention should be given to A. erecta in the Hawaiian Islands to monitor its possible expansion and competition with other psammophytic phototrophs in order to allow timely mitigation strategies if needed.…”

Present-Day Distribution and Potential Spread of the Invasive Green Alga Avrainvillea amadelpha Around the Main Hawaiian Islands

“…Our model indicated that regions experiencing at least 5 DHW during 2013-2016 were more suitable for A. amadelpha invasion. Cox et al (2017) speculate that recent warming events in 2015 caused a die-back in abundant native intertidal macroalgae, facilitating the rapid spread of leather mudweed in this intertidal environment. They observed an opportunistic increase in abundance of A. amadelpha following the reduction in brown algae (Padina sanctae-crucis and Dictyota spp.…”

“…They observed an opportunistic increase in abundance of A. amadelpha following the reduction in brown algae (Padina sanctae-crucis and Dictyota spp. ), which dominate multiple intertidal sites around Oahu (Cox et al, 2017). This invasion notably followed a period of unseasonably warm, calm seas and a series of severe bleaching events from 2014 to 2017.…”

“…One example of a particularly troublesome competitor that has garnered statewide attention in the past several decades is the invasive green macroalga Avrainvillea amadelpha. This macroalga, also known as "leather mudweed" due to its thick, paddle-like blades (Figure 1a), was first discovered in Hawaii in 1981 (Brostoff, 1989) and has been observed from the intertidal to the mesophotic zone (30-90 m) (Spalding, 2012;Pyle et al, 2016;Cox et al, 2017). Its origin and phylogeny remain uncertain, but it is not present in historical records of shallow marine habitats around the main Hawaiian Islands (MHI) (Brostoff, 1989).…”

Rarity and beta diversity assessment as tools for guiding conservation strategies in marine tropical subtidal communities

“…Although described as the richest marine habitats (Costello & Chaudhary, ), tropical coastal areas are still under‐studied (Cox, Spalding, & Foster, ; Kaehler & Williams, ) when compared to temperate shores or coral reefs (e.g., Mieszkowska et al, ). We performed descriptive analyses of diversity in marine subtidal rocky reef communities in a tropical region (sensu Spalding et al, ) of Rio de Janeiro state, Brazil.…”