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Secondary production connects primary producers with higher‐order consumers. The response of secondary production to seasonal variation in primary producers can influence trophic flows that underpin key ecosystem functions and services. Within the canopy‐forming macroalgal meadows of Ningaloo Reef, Western Australia, we quantified the secondary production generated by Sargassum‐associated epifauna across seasons and seascapes. We found a strong positive correlation between overall epifaunal production and Sargassum canopy size. Variation in epifaunal production was predominantly driven by seasonal changes in Sargassum canopy size. However, these seasonal effects were not uniform across the seascape. Key predictors of spatial and temporal variation in epifaunal production were the presence of invertivorous fish families, as well as daily and monthly fluctuations in sea temperature. Areal estimates of epifaunal productivity were much higher in summer than in winter, due to higher Sargassum canopy size and percent cover in summer. Epifaunal production was estimated to be more sensitive to Sargassum percentage cover than to canopy height. Modeling a 45% reduction in canopy height and percent cover of Sargassum associated with a marine heatwave event revealed a potential 81% drop in the areal rate of secondary production by Sargassum epifauna in this tropical fringing reef ecosystem. Disturbances to Sargassum canopy structure driven by global change can therefore significantly alter the productivity of these tropical macroalgal meadows and their ability to support higher level consumers within the food web, including important fisheries species. Our results highlight the importance of including epifaunal production estimates in predictive modeling when managing macroalgal‐dominated marine ecosystems. [ABSTRACT FROM AUTHOR]

Identifying and protecting nursery habitats for species is a key conservation strategy for the long-term sustainability of populations. In tropical ecosystems, macroalgal habitats have recently been identified as nurseries for fish of commercial and conservation significance. Here, we explore how local-scale variations in seaweed habitat quality interact with large-scale climatic conditions (Southern Oscillation Index, SOI) to influence the recruitment of three tropical fish species ( Lethrinus spp.), often targeted by fishers. New fish recruits and juveniles of all species were almost exclusively found in macroalgal nursery habitats, while adults of two of these species were predominantly found on adjacent coral reefs. Annual supply rates of new recruits were found to be strongly correlated to variations in the SOI, with La Nina conditions associated with higher recruitment. However, local rates of recruitment were generally poor predictors of older juvenile abundance. Instead, local juvenile abundance was more closely related to structural characteristics of macroalgae nursery habitat quality (density, canopy height, canopy cover) and/or predator biomass, at the time of survey, with species-specific habitat associations apparent. Given the dynamic nature of fish recruitment supply to the SOI, coupled with the effects of climatic and oceanic processes on the structure of macroalgal patches, these results suggest protection of macroalgal nursery habitats that maintain high canopy density, height and cover is critical to supporting the conservation of fish populations. [ABSTRACT FROM AUTHOR]

Using a functional approach, we quantified water motion and the distribution of fish swimming modes across five habitat zones and four exposure regimes commonly found on coral reefs. There were major spatial variations in net flow velocity (7.4-43.2 cm s-1) and rates of flow direction change (0.06-0.66 Hz) among habitats of different depth. Water motion within the shallow crest and flat habitats appeared to be largely wave driven, with rates of flow direction change (0.63-0.66 Hz) corresponding with approximately double the wave periodicity (0.31-0.36 Hz) and relatively little contribution (14-16%) from drift flow velocities. A similar variation in water motion was found among reefs of different exposure, with the highest and lowest velocities recorded in the exposed and sheltered locations, respectively. Exposed and oblique reef crests displayed greater temporal variation in wave height and water motion (24.4-59.5 cm s-1 net velocities) when compared with the relatively static conditions in sheltered (6.2-12.6 cm s-1) and lagoonal sites (17.1-25.3 cm s-1). We examined the distribution and functional structure of seven coral reef fish families (156 species) across the full range of water motion variation on these reefs. Pectoral-swimming fishes were the most abundant group, predominating in areas with high levels of water motion; pectoral-caudal- and caudal-swimming fishes displayed the opposite trend. We suggest that these differences are the result of an interaction between water motion and the biomechanical and energetic attributes associated with each swimming mode. [ABSTRACT FROM AUTHOR]

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, T antabiddi) were made during winter, spring, and late summer over a 26 month period. Canopy-forming 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-2 at Coral Bay) and lowest values in winter (down to 155 g fresh weight 0.25 m-2 at T antabiddi). 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. [ABSTRACT FROM AUTHOR]

We describe a mechanistic basis for maintaining an alternative degraded stable state on coral reefs: sediment-laden algal turfs. Using remote underwater video cameras we quantified rates of herbivory by coral reef fishes on epilithic algal turfs with natural and experimentally reduced sediment loads. Removal of sediment increased overall fish feeding rates 3.8-fold, and resulted in a decrease in mean algal turf length of 64% within 4 h. After 4 h, sediment accumulated in the treatment plots, but only returned to 41% of the original depth. A total of 20 species actively fed on the sediment removal plots, compared with 12 species in control plots. Of the five numerically abundant herbivorous fish species, all increased feeding by at least 225% in the absence of sediment. Only juvenile Scarus spp. fed to any extent (28% of bites) on control plots. We suggest that naturally occurring sediment loads in epilithic algal turfs can suppress herbivory and that sediment-laden algal turfs may be an alternative stable state on coral reefs. This may provide a mechanistic basis for the geological evidence of a sediment-induced turn-off of coral reef growth. With projected global sea-level rises due to climate change, reef-based sediment loads may be a critical factor in differentiating the relative resilience of coral reefs and identifying reef ecosystems that are at highest risk to rising sea levels. [ABSTRACT FROM AUTHOR]