Photosynthesis by benthic marine macroalgae makes an important contribution to the productivity of coastal seas. Quantification of photosynthesis and productivity of macroalgal assemblages is therefore important in understanding ecosystem functioning in coastal seas and providing realistic values for coastal productivity in global models. Estimates of macroalgal productivity are often based on the photosynthetic characteristics of thallus pieces or whole thalli, and not on those of communities. Such methods may overestimate rates of productivity as they do not account for neighborhood shading effects that may reduce photosynthetic rates in macroalgal stands that typically have high densities. In order to determine whether productivity estimates based on individuals differ from those based on communities, a controlled laboratory experiment was conducted with 3 dominant sub-canopy macroalgal species (Cystophora scalaris, Xiphophora gladiata and Undaria pinnatifida) from southern New Zealand. Photosynthetic parameters (initial slope of the photosynthesis vs. irradiance [P-E] curve α, saturation irradiance E k , maximum rate of photosynthesis P max and darkrespiration R d ) were obtained via P-E experiments using a custom-built respirometry chamber for a range of densities that corresponded to the minimum, average and maximum densities of these species in the field. A 5 to 7-fold decrease in P max was observed when the density of the algal stand was above 1 ind. m -2. R d and α were also lower in communities than for individuals. Results illustrate that estimates based on single specimens substantially overestimate productivity and we recommend that the densities used in experiments reflect those observed in the field.KEY WORDS: Density · Photosynthesis · Primary production · Respiration · Seaweed · New Zealand · Cystophora scalaris · Undaria pinnatifida · Xiphophora gladiata Resale or republication not permitted without written consent of the publisherAquat Biol 13: [41][42][43][44][45][46][47][48][49] 2011 P max is reached; R d ; and β, the level of photoinhibition at high irradiances (Falkowski & Raven 2007) (Table 1).The photosynthetic parameters derived from P-E curves can be used to predict rates of primary production at different irradiances (Falkowski & Raven 2007). The most accurate approach is to use whole seaweeds, so that they remain connected to their storage re serves, which, for large seaweeds such as species from the Orders Fucales and Laminariales, may be located in the holdfast and stipe, i.e. for entire individuals the carbon and nitrogen sinks are still accessible (Gevaert et al. 2008). Consequently, for structurally complex seaweeds, photosynthetic responses ob tained from thallus pieces versus intact individuals can be quite different (Bin zer & Middelboe 2005, Middelboe et al. 2006, Sand-Jensen et al. 2007. Similarly, for nutrient uptake by members of the Fucales, the rates of thallus sections can be up to 10 times higher than those of whole algae (Harrison & Druehl 1982, Hurd & ...
Spatially discrete stocks that vary in life history traits on a reef‐to‐reef scale present a challenge for fisheries managers. Nationally or regionally applied one‐size‐fits‐all approaches to management fail to account for this variability, and when coupled with fishing pressure this can result in serial depletion and localized extinction. Where mechanisms for local management exist, as they do in New Zealand via customary protection areas (Taiāpure Local Fisheries, Mātaitai Reserves), managers have the opportunity to trial tools on scales relevant to the stocks managed. This study assesses the effect of bag limits and a rāhui (temporary closure) in managing the spatially discrete, and culturally important, blackfoot abalone (Haliotis iris, pāua) in the East Otago Taiāpure, in southern New Zealand. Using surveys spanning 8 years and 26 sites, the rāhui was found to have had mixed results: halting the decline in pāua density at 0 m but not 0.5 m depths. The rāhui did, however, result in an increase in the percentage of individuals above the minimum legal size at both depths. Where bag limits alone had been applied, pāua densities and the percentage of legal‐sized individuals continued to decline over the 8‐year period, with the loss of large individuals, translating into a decline in estimated egg production. Growth estimates from mark–recapture and surveys of juvenile abundance at sites inside and outside the rāhui highlight the fine‐scale variability in pāua life history even within this local area. This suggests there may be biological impediments to pāua management using bag limits alone. Though pāua restoration efforts have had mixed results to date, this study highlights the potential for legally empowered local managers and communities to respond to fisheries declines in their own local area in an action‐driven, adaptable, and timely way.
The supply of dissolved inorganic carbon to seaweeds is a key factor regulating photosynthesis. Thinner diffusive boundary layers at the seaweed surface or greater seawater carbon dioxide (CO2) concentrations increase CO2 supply to the seaweed surface. This may benefit seaweeds by alleviating carbon limitation either via an increased supply of CO2 that is taken up by passive diffusion, or via the down-regulation of active carbon concentrating mechanisms (CCMs) that enable the utilization of the abundant ion bicarbonate (HCO3−). Laboratory experiments showed that a 5 times increase in water motion increases DIC uptake efficiency in both a non-CCM (Hymenena palmata, Rhodophyta) and CCM (Xiphophora gladiata, Phaeophyceae) seaweed. In a field survey, brown and green seaweeds with active-CCMs maintained their CCM activity under diverse conditions of water motion. Whereas red seaweeds exhibited flexible photosynthetic rates depending on CO2 availability, and species switched from a non-CCM strategy in wave-exposed sites to an active-CCM strategy in sheltered sites where mass transfer of CO2 would be reduced. 97–99% of the seaweed assemblages at both wave-sheltered and exposed sites consisted of active-CCM species. Variable sensitivities to external CO2 would drive different responses to increasing CO2 availability, although dominance of the CCM-strategy suggests this will have minimal impact within shallow seaweed assemblages.
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