Spatio-temporal variability in the deposition of beach-cast kelp (wrack) and inter-specific differences in degradation rates

Abstract

Coastal habitats dominated by marine macroalgae typically exhibit high rates of primary productivity and play a key role in local and regional carbon cycles and stores. In temperate regions, large brown algae (i.e. kelps and fucoids) contribute significantly to macroalgal primary production, most of which is exported from source habitats as detritus. The ultimate fate of this detritus and the processes controlling detrital pathways into food webs and carbon cycles remain poorly understood. Based on field surveys, we quantified the biomass of kelp-derived detritus (wrack) at sandy and pebble-dominated shores in Ireland and conducted a manipulative field experiment to test for inter-specific differences in detritus degradation rates and the effect of macroinvertebrate detritivores. Overall, accumulated wrack biomass was similar on all shores but varied temporally depending on habitat type. Degradation rates and the nutritional (C:N) and chemical (polyphenol concentrations) properties differed among kelp species. Interestingly, exclusion of macroinvertebrate detritivores did not affect kelp degradation rates, C:N ratios or polyphenol content. Our findings show that rates of macroalgal breakdown differ among kelp species and that, in contrast to other aquatic systems, macroinvertebrates appear to play a very limited role in the breakdown of these marine detrital subsidies, suggesting a key role for meiofauna and microbes in detritus processing. Increasing recognition for the role of detritus in coastal food webs and carbon cycles warrants a better understanding of the mechanisms underpinning degradation rates.

Examining the production, export, and immediate fate of kelp detritus on open‐coast subtidal reefs in the Northeast Atlantic

“…This estimate is likely to be broadly representative of L. hyperborea populations across its range in the NE Atlantic, as our northern populations are similar in structure to those in Norway and our southernmost populations akin to those in France and Spain (Jupp and Drew 1974; Pedersen et al 2012; Pessarrodona et al 2018). These findings support the emerging role of L. hyperborea forests as key donors of organic matter to a range of recipient habitats in the NE Atlantic (Smale et al 2018; Ramirez‐Llodra et al 2021; Gilson et al 2021 a ), and advance our understanding of rates of detrital release, export, and persistence along dynamic wave‐exposed coastlines dominated by extensive kelp forests.…”

Wrack line formation and composition on shores of a large Alpine lake: The role of littoral topography and wave exposure

“…In general, wave energy dissipation rates are higher over immobile, rough bed forms than over mobile sandy lake beds or mudflats [115]. In line with this Gilson et al [97] found a generally lower wrack biomass on sandy sites than on pebble shores of marine tidal coasts in NW Ireland.…”

“…The abundance of wrack can be quantified by visual estimation of cover percentage [ 39 , 96 , 97 ], by measuring its width and height [ 31 ], by calculating its volume [ 40 ], or by taking representative samples and calculating the wet weight, dry weight or ash-free dry weight [ 32 , 35 , 40 , 98 ]. We used the wrack width and height to capture variability across sites in an efficient way.…”

“…beach reflectivity [37] not included in the model beach morphology (bays, horns) [39,114] not relevant no such rhythmic structures cross-shore profile, inclination [35,36,40] shore relief (WEU) beach substratum and hydraulic roughness [31,97] not included in the model seasonality winds (velocity, direction) [31,36] not included in the model the same strong wind events for the lower wrack line of all sites wave forces [31,32] not included in the model depending on seasonality of winds (s. above) water level change not relevant not included in the model identical for all sites https://doi.org/10.1371/journal.pone.0294752.t004…”

Seasonal and spatial variability in rates of primary production and detritus release by intertidal stands of Laminaria digitata and Saccharina latissima on wave‐exposed shores in the northeast Atlantic

“…The majority of kelp‐derived production (>80%) enters the food web through detrital pathways, with high rates of export from source populations and the potential for long‐distance transport to recipient ecosystems (Krumhansl & Scheibling, 2012 ). This transfer of carbon has been shown to constitute a crucial trophic subsidy in a range of habitats, including rocky shores, sandy beaches, submarine canyons, and the deep‐sea (Gilson, Smale, Burrows, et al, 2021 ; Krumhansl & Scheibling, 2012 ; Polis et al, 1997 ). Detrital production is generated by two primary mechanisms, chronic erosion of material (typically from the distal part of the blade) or dislodgment of sections or entire thalli (de Bettignies et al, 2013 ; Krumhansl & Scheibling, 2011 ).…”