Light Limitation within Southern New Zealand Kelp Forest Communities

April Light Limitation within Southern New Zealand Kelp Forest Communities Matthew J. Desmond 0 1 2 Daniel W. Pritchard 0 1 2 Christopher D. Hepburn 0 1 2 0 Department of Marine Science, University of Otago , Dunedin , New Zealand 1 Funding: This study was funded in part by a Tertiary Education Commission scholarship to MJD. All additional funding was provided by the University of Otago. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript 2 Academic Editor: Judi Hewitt, University of Waikato, NEW ZEALAND Light is the fundamental driver of primary productivity in the marine environment. Reduced light availability has the potential to alter the distribution, community composition, and productivity of key benthic primary producers, potentially reducing habitat and energy provision to coastal food webs. We compared the underwater light environment of macroalgal dominated shallow subtidal rocky reef habitats on a coastline modified by human activities with a coastline of forested catchments. Key metrics describing the availability of photosynthetically active radiation (PAR) were determined over 295 days and were related to macroalgal depth distribution, community composition, and standing biomass patterns, which were recorded seasonally. Light attenuation was more than twice as high in shallow subtidal zones along the modified coast. Macroalgal biomass was 2-5 times greater within forested sites, and even in shallow water (2m) a significant difference in biomass was observed. Longterm light dose provided the best explanation for differences in observed biomass between modified and forested coasts, with light availability over the study period differing by 60 and 90 mol photons m2 at 2 and 10 metres, respectively. Higher biomass on the forested coast was driven by the presence of larger individuals rather than species diversity or density. This study suggests that commonly used metrics such as species diversity and density are not as sensitive as direct measures of biomass when detecting the effects of light limitation within macroalgal communities. - Competing Interests: The authors have declared that no competing interests exist. The availability of photosynthetically active radiation (PAR), henceforth referred to as light, in the worlds coastal seas is highly variable and in many cases substantially modified by anthropogenic activities [1,2]. The discharge of wastewater, dumping of dredge spoil and the clearance of land for agricultural, horticultural and urban purposes all increase sediment loading, and as a result turbidity, in coastal seas [25]. Light availability acts in concert with nutrients [6], temperature [7], herbivory [8] and wave exposure [9] to drive photosynthetic carbon fixation and ultimately, coastal primary production [1012]. Changes to the light environment of coastal seas could have major implications for the productivity of coastal food webs which support an estimated 90% of the worlds fisheries [13] and therefore, light availability deserves closer investigation [10,14,15]. Macroalgae are major primary producers in coastal seas, in some cases they provide up to 90% of total carbon to coastal food-webs [1618]. Macroalgae are also considered foundation species or ecosystem engineers as they form complex three dimensional habitat [7,19]. This habitat supports an array of organisms through a range of life history stages and provides numerous ecosystem services [7,20,21]. Assuming the presence of suitable substrate and lack of significant grazing pressure, light penetration into the water column ultimately controls macroalgal depth distribution, and as a result potential primary productivity and community structure [17,22]. Productivity and overall ecosystem functioning of macroalgal communities is therefore greatly affected by changes in the underwater light environment [3,2328]. Globally macroalgal distribution has been estimated to be limited by light in 3458% of the non-polar coastal regions [17]. With predicted increases in coastal anthropogenic activity this estimate is likely to increase [1,29]. Macroalgae employ a range of mechanisms to deal with variability in the quality and quantity of light reaching the benthos [3,28,30,31]. Responses of macroalgae to such variability can occur as regulation, e.g. dynamic photoinhibition in periods of high light stress [32,33], acclimation, e.g. varying concentrations of photosynthetic pigments [3436], and / or adaption, e.g. changes to thallus morphology which alter the efficiency of light absorption per unit of photosynthetic tissue [32,37]. However, changes in turbidity, especially those caused by anthropogenic disturbance, can occur over relatively short time scales and in some instances species may not have the capacity to respond [38]. Light limitation has the potential to compress depth distributions of species [26,37,39], reduce growth rates [25,28] and decrease community complexity [40]. All of these changes culminate in a reduction of primary productivity and habitat availability as well as decreased ecosystem resilience to stress [41,42]. There is consensus that in-depth, long term investigations are critical to better understand how the underwater light environment influences the productivity and structure of such important coastal marine ecosystems [3,10,14,15,40,43] and there is growing concern regarding the loss of macroalgal dominated habitats worldwide [40,44]. The focus of this study was to quantify light in two shallow coastal reef ecosystems with varying underwater light regimes. This was achieved by comparing subtidal reef environments in southern New Zealand. One is associated with catchments containing intact mixed native podocarp forest (termed forested) on Stewart Island (Rakiura) and the other is a coast dominated by agriculture, forestry and urban development (termed modified) in East Otago (Fig 1). Mixed podocarp forests on Stewart Island are similar to those that once covered many coastal areas within New Zealand, in particular the southeast of the South Island which includes the East Otago region [4547]. Data gathered were used to address the question: Does the availability of light explain coastal patterns in benthic macroalgal depth distribution, community composition and standing biomass? We hypothesised that sites associated with the forested catchment will receive a higher annual light dose and support greater macroalgal biomass, a more complex community structure and deeper depth limits for macroalgal species compared to sites associated with the modified catchment. This information is important in understanding processes controlling coastal primary productivity and can potentially be applied to support coastal and marine management initiatives. Field studies did not require any permit or permission and did not involve any protected or endangered species. This study employed a nested design by (...truncated)