Effects of turbidity on habitat-forming seaweeds in Southern New Zealand.

Abstract

Even though coastal zones represent only 15 percent of the world’s land-surface area, they accommodate three times more people than the global average. The ever-increasing demand for resources and space lead consequently to increased pressure on these systems through pollution, deforestation and discard of sewage and fertiliser. Subtidal seaweed forests are affected by many of those factors. They are among the most diverse and productive ecosystems on this planet and provide many ecosystem services based on primary productivity and the flow of energy through myriad species, of which many are economically important. Over the last decade, complete losses or significant declines in seaweed health and abundance have been documented in many parts of the world that could be linked to anthropogenic activities and a changing climate. One factor which is often overlooked but has a major effect on the health, distribution and primary productivity levels of seaweeds is an increased load of suspended sediments. These are caused by a range of natural and anthropogenic coastal and terrestrial processes and lead to a reduced light environment in nearshore waters. Although there is a sound understanding regarding the processes involved in photosynthesis and the ways macroalgae respond to light availability, there is a lack of knowledge about acclimation thresholds, acclimation, and recovery time as well as site-specific application of this knowledge and its interactions with other stressors. Furthermore, many studies are focused on individual species but comparative studies of physiological responses of species sharing the same habitat and filling the same functional niche are rare. In this study, the focus was on confirming light availability as one of the main drivers for macroalgae distribution and abundance around Lyttelton Harbour, Christchurch, and to investigate physiological processes underpinning seaweeds’ ability to adjust to different degrees of turbidity. Resistance and resilience capacities to long periods of light limitation as well as vulnerability to additional thermal-stress were tested in order to identify tolerance thresholds and to determine differences between species. Habitat-forming species belonging to the Orders of Laminariales and Fucales were investigated in this research as they are foundation and keystone species in rocky reef communities and their resilience and resistance during disturbances determines the fate of the whole ecosystem. Using biological survey data combined with satellite derived environmental parameters, I identified the main drivers for the distribution of the most common species in Lyttelton Harbour as the degree of exposure to wave forces as well as average light availability. Maximum sea surface temperatures during summer months further contributed to the absence of the kelp Ecklonia radiata. Results from laboratory-based indoor and outdoor microcosm experiments further showed that some species are disproportionately affected by reduced light availability and are less resistant or resilient to abrupt, long-lasting light unavailability than others. Ecklonia radiata, Macrocystis pyrifera and Undaria pinnatifida expressed higher photosynthetic efficiencies when adapted to low light, however this observation could not be made across seasons. Highest efficiencies and maximum capacities were observed during winter in U. pinnatifida, with rates 2 to 10-fold higher than of other investigated species and especially pronounced when low-light adapted. When light availability was abruptly restricted for an extensive period, however, Undaria was not be able to survive. All other species on the other hand were able to maintain basic functions during short and long light-limitation periods and most were adjusted quickly to new light conditions. Recovery time was slower when light restriction was longer. With increased water temperatures, seaweeds expressed an increased respiration, which ultimately resulted in decreased primary productivity levels. This observation was especially pronounced in the investigated laminarians also exposed to low-light, which highlights their vulnerability to heat stress when occurring in turbid environments such as the inner Lyttelton Harbour area. The findings of this study contribute to a better understanding of species- specific responses to changing light availability and can be used for more precise predictions of seaweed distribution, changes in community structure and productivity under future environmental changes. They further suggest that the investigated fucoid species are more robust against light-limitation and thermal stress compared to the Laminarians. However, extraordinary performances measured for the invasive seaweed U. pinnatifida highlight its capability to adjust well to a varying light environment, also its vulnerability to extended periods of light-limitation and heat stress. This thesis provides evidence that suggests a high degree of inter-and intra- specific variability in the abilities of seaweeds to acclimate to, or persist in, variable and limiting light conditions. The understanding of fundamental physiological processes and adjustment limitations of species occurring in the same habitat and filling the same functional niche is essential for proper ecosystem management that can ensure sustainable ecosystem functioning now and in the future.