Sea temperature shapes seasonal fluctuations in seaweed biomass within the Ningaloo coral reef ecosystem

Abstract

Environmental drivers of seaweed biomass were investigated at Ningaloo, Western Australia, a coral reef ecosystem with negligible anthropogenic influences on seaweeds from fishing, farming, or eutrophication. Periodic surveys of benthic macroalgae occupying seaweed-dominated beds within the lagoon at two locations (Coral Bay, Tantabiddi) were made during winter, spring, and late summer over a 26 month period. Canopyforming Sargassum spp. biomass fluctuated over a seasonal growth-decay cycle, with highest values in the warm summer months (up to 1013 g fresh weight 0.25 m 22 at Coral Bay) and lowest values in winter (down to 155 g fresh weight 0.25 m 22 at Tantabiddi). Conversely, prominent understory seaweed genera Dictyopteris and Lobophora reached peak biomass in winter, when the Sargassum spp. canopy was lowest. Sargassum spp. biomass variation could be attributed largely to time (52%), location (21%), and site (26%), with low variation within individual seaweed beds (1%). Statistical analysis of the influence of five environmental variables (temperature, light, wind-driven upwelling, rainfall, significant wave height) indicated that location and sea temperature (1 month antecedent to biomass) provided the best explanation for Sargassum spp. biomass fluctuations. While sea temperature is a key driver of seaweed temporal dynamics, heterogeneity at the kilometer scale suggests that spatial context is also important. Given the important role of seaweeds in many ecosystem processes, this strong biophysical coupling between Sargassum spp. biomass and sea temperature suggests that thermal climate change will significantly affect coral reef productivity and biodiversity.

Seasonal variation ofSargassum ilicifolium(Phaeophyceae) growth on equatorial coral reefs

“…, Fulton et al. ). Seawater temperature was recorded daily at all reefs with the exception of TPL using HOBO Water Temperature Pro v2 data loggers (±0.2°C accuracy).…”

“…, Fulton et al. ). Among these environmental variables, seawater temperature is often considered the most important for driving Sargassum seasonal growth and reproductive cycles, although its influence varies across regions (Table ).…”

The Dynamics of Coral-Algal Interactions in Space and Time on the Southern Great Barrier Reef

“…Spatially, macroalgae display distinct within and between reef patterns in biomass and community composition (Diaz-Pulido et al, 2007;Wismer et al, 2009). Macroalgae also show marked seasonal dynamics, primarily due to strong seasonal oscillations in temperature and light (Glenn et al, 1990;Ateweberhan et al, 2006;Fulton et al, 2014). The effects of spatio-temporal variability on tropical macroalgae, however, have mostly been inferred from the occurrence of seasonal peaks and have principally focused on large, conspicuous species (i.e., Sargassum) that bloom in the austral summer (Vuki and Price, 1994;McCook, 1997;Lefèvre and Bellwood, 2010).…”

Chemically cued suppression of coral reef resilience: Where is the tipping point?

“…If seaweed cue concentration influences whether a habitat will attract or repel juveniles, annual fluctuations in seaweed growth have the potential to deter larvae from settling into otherwise acceptable habitats if blooms overlap with periods of peak larval recruitment. For instance, in the tropical Indo-Pacific, fish recruitment and abundance of brown macroalgae such as Sargassum , Turbinaria , and Dictyota both peak during the warm summer months (Ferrari et al 2012; Fulton et al 2014). When such overlaps occur, ecologically critical fishes could fail to recruit and help restore reef resilience even if habitat quality is high during portions of the year when macroalgae senesce and reach seasonal lows.…”