Dynamics of productivity in naturally structured macroalgal assemblages: importance of canopy structure on light-use efficiency

Abstract

Macroalgae are undoubtedly a major supplier of organic carbon to marine food webs, yet few studies have attempted to examine primary production in complex macroalgal communities. Many studies have tested the photosynthetic characteristics of single species with little attention given to the associated algal assemblages. The role of canopy structure and species diversity in these assemblages will help in understanding the role of biodiversity in enhancing ecosystem function. In the present study, photosynthesis-irradiance relationships (P-E curves) of intact intertidal algal assemblages showed no signs of saturation at high irradiance levels, as is typically seen in singlespecies curves. Canopy-forming species appear to play a vital role in enhancing overall assemblage net photosynthesis, particularly at high irradiances (>1500 µmol m -2 s -1 ), with the loss of canopy species causing up to a 50% decline in the photosynthetic rate of assemblages. There is also evidence that the mechanisms of light use are significantly different in intact assemblages because they do not act as the sum of their parts at high irradiance levels. Furthermore, with greater structural complexity, net photosynthesis of the assemblage is enhanced, particularly at high irradiance. The present study demonstrates the importance of using realistic species compositions and assemblage structure to identify the potential mechanisms enhancing function with increasing diversity. A better understanding of light-use efficiency within macroalgal communities underpins the role of biodiversity and community structure in these ecosystems and the potential effects of species loss on various forms of ecosystem function.KEY WORDS: Macroalgae · Net photosynthesis · Assemblage · Irradiance · Canopy Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 421: [97][98][99][100][101][102][103][104][105][106][107] 2011 sonal biomass accumulation, in situ measurements of frond elongation or frond area increments fail to take account of processes such as abrasion, herbivory, reproduction, and sloughing in estimations of primary productivity (Larkum 1986). Likewise, short-term incubations measuring carbon assimilation (or oxygen production) are often performed using excised tissue of macroalgal specimens (Flores-Moya et al. 1995, Gómez et al. 1997 and extrapolating these to whole plants. Furthermore, laboratory measurements of primary production in single specimens often fail to account for inter-and intra-species interactions among algal species that occur under natural conditions (Middelboe & Binzer 2004). Recent research has indicated the important influence of algal density and diversity on the use of light (Binzer & Sand-Jensen 2002, Middelboe & Binzer 2004 and reveals the problems associated with measuring productivity in single specimens or excised tissue. Light attenuation through canopy species results in greater filtration of light and, therefore, a more linear relationship between productivity and irr...

Metabolic activity of intertidal Fucus spp. communities: evidence for high aerial carbon fluxes displaying seasonal variability

“…37 . On the contrary, Tait and Schiel (2011) demonstrated that the structural 54 differences of primary producers, but also their location in the community assemblage could 55 lead to an enhancement of the global primary production. Therefore, all these interactions 56 play a fundamental role and need to be accounted for when investigating community 57 metabolism, as highlighted by Migné et al (2015a).…”

Photosynthetic response of monospecific macroalgal stands to density

“…(1) did not model the data at either low or high PFDs. A similar response was recorded for a mixed-canopy of Cysto phora tortulosa and Hormosira banksii, in which case P max was not achieved at PFDs of 2000 µmol photons m -2 s -1 (Tait & Schiel 2011); those authors were also unable to apply 'traditional' P vs. E models to their data. The explanation for these trends is that when growing in canopies, seaweeds self-shade: while the uppermost blades in a canopy may reach P max , or even exhibit photoinhibition, the lower blades will receive much less light and exhibit un-saturated rates of photosynthesis (Middelboe & Binzer 2004).…”

“…m -2 for Xiphophora was not seen at 41 ind. m -2 , but it is possible that if we had used higher PFDs we may have observed a pattern similar to Tait & Schiel (2011) and to that for the 13 ind. m -2 treatment in this study.…”

Photosynthetic activity and productivity of intertidal macroalgae: In situ measurements, from thallus to community scale

“…Although individual photosynthesis must influence the community response, individual metabolism was not expected to be closely related to community metabolism, because this represents a complex integrated response of many different species exposed to a wide range of local light conditions within the canopies (Middelboe and Binzer, 2004;Binzer and Middelboe, 2005;Middelboe et al, 2006). Tait and Schiel (2011) showed that layering within macroalgal assemblages plays a major role in maintaining net photosynthesis throughout the natural range of irradiance and even enhances photosynthesis at high irradiance. Tait and Schiel (2010) measured the underwater productivity of three fucoid assemblages.…”

“…Johnson et al, 1974;Quadir et al, 1979;Dring and Brown, 1982). Since then, differences between some shallow-water macroalgae species have been shown to be smaller at the communities scale than experiments on single thallus pieces suggested (Binzer and Middelboe, 2005) and an increasing productivity of fucoid assemblages down a shore-height gradient has been shown (Tait and Schiel, 2011). These community-scale measurements were limited, however, to immerged conditions and trends might be different under peculiar conditions of the emersion periods.…”