Estimating productivity in habitat-forming seaweeds

Macroalgal beds provide the ecological foundations for most shallow reef ecosystems in temperate environments. With distinctive canopies primarily of brown laminarian algae (northern hemisphere), or laminarian or fucalean algae (southern hemisphere), in many areas these habitats are at risk from human activity. Overexploitation, pollution, and other effects of coastal activities have resulted in significant habitat loss in coastal ecosystems, and human-induced climate change is now seen as a major threat to ecosystem health in marine systems. Understanding the impact of climate change is particularly important for habitat-forming ecosystem engineers like kelps, as these species form the basis of hierarchically organised communities and play a fundamental role in determining community structure and ecological processes. South eastern Australia has experienced increases in marine temperatures at nearly four times the global average, and there is now evidence that, in some locations, macroalgae communities are retreating in a manner consistent with ocean warming. Successful management of marine systems requires understanding ecosystem processes, particularly the patterns and magnitude of production. Macroalgal communities often show relatively low resistance to disturbance, yet rapid recovery once disturbances are removed, hence they are generally highly dynamic in response to environmental perturbations. As a result, macroalgae are likely to play an increasingly important role in buffering the short term/dynamic effects of climate change on temperate reef communities. Knowledge of the productivity of seaweed-dominated temperate reef systems is largely a synthesis from studies conducted over small spatial scales utilising a variety of methods that generally measure different characteristics of both individual seaweeds and collectively. As a result of the diversity of measurement methods, estimates of gross primary productivity (GPP), production potential, and macroalgal biomass for temperate reefs are numerous and variable. This can lead to challenges for ecologists attempting to amalgamate research findings to facilitate long-term, broad-scale perspectives or compare short-term research between spatially separated communities. However, to date there has been relatively little research to compare measurement approaches and quantify differences in productivity estimates across the different techniques. The present research provides a unique investigation into some of the techniques and methodology involved in measuring primary productivity in marine systems, particularly kelp forests, using the macroalgae Ecklonia radiata, Phyllospora comosa and Macrocystis pyrifera as study species. The work is based on both field and laboratory exploration of productivity measurements and associated parameters. In situ measurements of primary productivity (diel oxygen modelling, benthic oxygen exchange chambers) or PSII electron transport (PAM fluorometry) are compared, and the possibility of using acoustics as a means of quantifying oxygen production at large scales is explored, as has already been applied in seagrass beds. This thesis also provides an in depth investigation of the effect of variability in sampling methodology with regards to interpretation of PAM fluorometry-derived parameters. Chapter 2 investigates the acoustic properties of Ecklonia radiata. The density, sound speed and resulting adabiatic compressibility of E. radiata tissue were investigated in the laboratory. Four methods were developed and trialled to determine the intrinsic sound speed of Ecklonia radiata tissue based on measurement of the time of flight of an ultrasonic pulse, while compressibility was calculated from density measurements. The results show that Ecklonia radiata sound speed and density are higher, and compressibility lower, than that of seawater. Properties varied according to size and tissue type and the variation likely reflected differences in cell type, packing and structure as well as the concentrations of alginates and other carbohydrates. These are important considerations for acoustic propagation and the results provide valuable inputs for future acoustic work. Chapter 3 focuses on the acoustic modelling of different scenarios of primary production in a shallow water rocky reef environment of Fortescue Bay (Canoe Bay), Tasmania, where E. radiata dominates the canopy. In February 2012, the environment was continuously probed by acoustic signal transmission and monitored by a comprehensive set of oceanographic sensors with the aim to assess the potential for acoustics to quantify excess oxygen production in bubble form. Ray-theory acoustic modelling results indicate that ecologically-significant void fractions of oxygen in the canopy layer from production would be clearly seen in diel variation of propagation features such as the energy decay rate of the medium impulse response. The model can then be used to invert empirical data for retrieving void fraction. However, comparative analysis of part of FORTES 12 data and model suggests that no large excess of bubbles was produced by photosynthesis under the present environmental conditions, in contrast to earlier observations made in seagrasses. As a result, the use of acoustics as a means of measuring primary productivity in kelp could not be further explored during the course of this research. Chapter 4 provides a unique comparison of the estimates of photosynthetic O\(^2\) production rates in an Ecklonia radiata dominated community using three different measurement methods: diel oxygen GPP models, benthic oxygen exchange chambers, and electron transport rate from PAM fluorometry which is usually interpreted as a measure of production potential. All three methods were run concurrently in situ in Fortescue Bay, Tasmania. The first diel oxygen model was fitted to in situ measures of dissolved oxygen (DO) in the environment and demonstrated a good fit, however, a consequence of this approach is that large variation in oxygen production was predicted at low PAR levels. A second model was created which utilised an explicit relationship between DO production and PAR, but it didn’t represent DO at the surface as well as the first model. Importantly, the two models indicate similar daily production rates of the seaweed bed (all species combined) that are ~ 2 times that predicted for the kelp alone based on incubations in the benthic chambers and scaling for the average size of adult kelp sporophytes and their population density. Oxygen evolution from incubation of sporophytes in benthic chambers and PAM fluorometry derived electron transport rates showed similar patterns, but the results indicate that the latter method may overestimate potential photosynthesis. The results suggest that diel oxygen modelling, benthic oxygen exchange chambers and PAM fluorescence can all provide good indications of productivity in shallow water marine environments. However, care must be taken in interpretation of results as each method differs in the type of productivity estimates it provides. As a direct measure of total seaweed production per unit area of reef, estimates from models based on empirical measures of environmental DO have much to recommend them. Chapter 5 details a final analysis investigating the effects of diurnal, seasonal and latitudinal variability in ambient light on PAM-derived parameters, as well as possible effects associated with depth, within- and between-alga variation in PSII performance, and latitudinal effects unrelated to the light climate. This research was based on field measurements undertaken in Tasmania, Western Australia and New South Wales, Australia in both summer and winter during 2012 and 2013, focussing on Ecklonia radiata, Macrocystis pyrifera and Phyllospora comosa. Photosynthetic characteristics of all species were highly dependent on the time of day, depth, latitude/region, season, and part of the thallus from which measurements were taken. Patterns dependent on time-of-day, depth and thalli placement varied with season and/or geographic region, and the nature of these patterns varied between species. It is clear from this work that efforts to standardise approaches to taking measurements of seaweeds using PAM fluorometry will be essential if measurements are to be compared meaningfully across studies. The key findings of this thesis are: (1) a first determination of the acoustic properties of E. radiata tissue which enable the development of scattering models to interpret scientific echosounder data collected in kelp beds; (2) a Gaussian beam/finite element beam code (Bellhop) with detailed environmental input and a huge number of beams can predict the acoustic character of a shallow water rocky reef and bubble layers with low-frequency effective sound speed; (3) the model allows prediction of the acoustic energy decay rates due to various scenarios of ecologically-relevant photosynthetic O\(^2\) production rates; (4) day vs night acoustic measurement and model data comparisons challenge void fraction predictions made from well-established measurements and methods; (5) diel oxygen modelling, benthic oxygen exchange chambers and PAM fluorescence can all provide good indications of productivity, however, understanding the limitations of each method is essential when interpreting the results as the measurements they provide are not directly comparable; and (6) applying a consistent sampling methodology is a key consideration when planning research utilising PAM fluorometry as diurnal, seasonal, and latitudinal variability, as well as effects associated with depth and within- and between-alga variation in PSII performance will have significant impact on PAM-derived parameters. The results of this work give valuable insight into the advantages and disadvantages involved with several main techniques currently utilised to measure production of macroalgal/seagrass beds, and the challenges faced by ecologists attempting to interpret results and compare research between methods and across studies. Last but not least, this study provides important and relevant information on the potential use of acoustics as a future means of determining productivity of benthic habitat on large scales in marine environments. The work presented herein will assist in both development and interpretation of future study of productivity in marine systems