Shears, NRees, ARodgers, Kirsten2015-04-2120142014https://hdl.handle.net/2292/25270Habitat-forming kelps are important primary producers on temperate reefs worldwide, yet obtaining accurate measurements of in situ rates of photosynthesis and respiration for kelp is challenging. There is limited information about the drivers of spatial and temporal variation in net primary production (NPP), and little is known about how they will respond to climate change impacts such as warming temperatures in combination with additional stressors. Some of these information gaps are addressed in this thesis for the important habitat-forming kelp Ecklonia radiata. The primary objectives were to: (1) develop an inexpensive photorespirometry system that can be used to measure photosynthesis and respiration rates and generate photosynthetic-irradiance (P-E) curves for entire adult kelp in situ, (2) investigate season and depth-related variation in P-E curves, (3) estimate primary production (NPP) of E. radiata kelp forest using a physiologically-based model of benthic macroalgal production that incorporates in situ estimates of P-E parameters, biomass and light, and (4) investigate the interactive effects of elevated temperature and reduced light (simulating reduced water clarity) on the P-E response, productivity and resilience of adult E. radiata. The photorespirometry system that was developed consists of a polyethylene plastic bag and frame that encloses the lamina of the kelp and seals around the stipe. Water is circulated through the chamber by a closed pump system and oxygen evolution or consumption is recorded by an enclosed logger. By varying the irradiance in the chamber with shade cloth, and utilizing the variation in light associated with depth, the photosynthetic rates of an individual can be measured over a range of irradiances in situ. Using this method laboratory-derived P-E curves were compared to those obtained in situ. P-E curves from individuals measured in situ differed from those measured in the laboratory, with maximum photosynthetic rates in the laboratory being 42% and 55% lower under natural and artificial light respectively. In situ measurements of photosynthesis-irradiance relationships at two depths (6 and 14 m) over the annual cycle found that E. radiata at 14 m had higher maximum photosynthetic rates (Pmax) and photosynthetic efficiency (alpha) than kelp at 6 m. Kelp at both depths had greater photosynthetic efficiency (alpha) and reduced compensating irradiance (Ec) in winter compared to summer. Seasonal and depth-related variation in NPP of E. radiata was examined using a physiological model that incorporates in situ estimates of P-E parameters, biomass and light. Seasonal productivity patterns followed seasonal patterns of irradiance and biomass, with the highest NPP coinciding with the peak in biomass and irradiance in summer, and the minima in winter. Estimated annual production at 6 m (615 ± 7 g C m-2 y-1)was 1.6 times greater than at 14 m (374 ± 4 g C m-2 y-1). This was proportional to the difference in biomass of E. radiata between depths, indicating that the greater photosynthetic abilities of deeper kelp offset the effects of reduced light on overall NPP at depth. Temperature and light were manipulated in a laboratory-based mesocosm experiment and P-E curves, growth, and survival of adult kelp were measured among treatments. Kelp were also subjected to a disturbance (a period without light, simulating a large storm event), to investigate how the temperature and light treatments affected resilience. An increase in maximum water temperature (+3°C) did not greatly alter the photosynthetic response of E. radiata, but reduction in light levels had negative effects on photosynthetic capacity and rates of tissue loss. Following the disturbance, kelp had higher mortality and reduced ability to recover in the warmer treatments. These effects of increased temperature were greatest in low light treatments. To examine the overall effects on kelp productivity, the experimental findings were incorporated into a physiologically-based model of production. This predicted that under warmer conditions (+3°C) daily net primary production (NPP) in summer would be reduced by 17% at ambient light levels and 89% at low light levels. At ambient temperatures NPP was reduced by 38% under low light conditions. Following disturbance these effects were exacerbated, with negative estimates of daily NPP for both warm treatments and under reduced light at ambient temperatures. The chamber system described here provides, for the first time, a method for generating P-E curves and estimating photosynthetic parameters for entire adult kelp in situ. It can be adapted for use on other kelps and large macroalgae, and used in tank-based experiments. Importantly, the findings also indicate that estimates of productivity based on laboratory-derived photosynthesis measurements can underestimate kelp forest productivity. This research provides the first in situ P-E curves of adult kelp over the annual cycle and across multiple depths, and demonstrates how this information can be incorporated with biomass and light data in a physiologically-based model to estimate NPP of kelp. The findings highlight the importance of understanding depth-related variation in photosynthetic performance when estimating primary production of subsurface kelp forests. The results from the laboratory experiment using the chamber system to quantify the effects of elevated temperature, reduced light, and disturbance on photosynthetic performance suggest that while kelp can likely tolerate some level of warming or reduction in light, under such conditions they would be less resilient to additional perturbations and large declines in kelp productivity would be expected. Kelp forests are under threat from a variety of stressors, perhaps most importantly from climate change, and in many cases very little is known (or quantified) about many of the important services that kelp forests provide. The methods and data presented in this thesis have provided an increased understanding of E. radiata productivity and ecological function and how this important habitat-forming kelp may be impacted by climate change.Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated. Previously published items are made available in accordance with the copyright policy of the publisher.https://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htmhttps://creativecommons.org/licenses/by-nc-nd/3.0/nz/ThesisCopyright: The Authorhttp://purl.org/eprint/accessRights/OpenAccessQ112906871