Environmental stress in coastal ecosystems: increased temperature and sedimentation effects on an intertidal fish

Coastal ecosystems are undergoing rapid and intensifying environmental change due to increasing anthropogenic pressures and climate change. Marine organisms inhabiting these dynamic ecosystems are exposed to multiple stressors, including elevated temperature, increased turbidity, and rising pollutant loads, which can influence their physiological performance, behaviour, and long-term resilience. In this thesis, across complementary field studies, laboratory experiments, and ecosystem modelling, I investigate how temperature and suspended sediments affect the physiology, behaviour, and ecology of the common triplefin, Forsterygion lapillum, a widespread and abundant intertidal fish species in New Zealand. In Chapter 2, I conducted field observations revealing that F. lapillum diet, body condition, and abundance vary substantially across seasons and between neighbouring coastal habitats. Warmer temperatures were consistently associated with better body condition, greater abundance, and increased gut volumes, reflecting temperature-driven changes in feeding activity. Sporadic measurements of turbidity showed no consistent effect on fish condition or abundance. Macroinvertebrate communities differed between areas and seasons, and these differences were reflected by shifts in F. lapillum diet and prey selectivity, suggesting that prey availability and quality are key drivers of feeding patterns in natural populations. In Chapter 3, I ran short-term behavioural experiments, which demonstrated that increased turbidity reduces foraging efficiency, with fish exhibiting slower strike speeds under high turbidity, indicating reliance on visual cues for prey detection and subsequent impaired feeding in sedimented waters. Conversely, routine and burst swimming speeds were unaffected by elevated temperature and turbidity, indicating behavioural plasticity in F. lapillum, and an ability to maintain movement performance under short-term stress. An acute thermal ramping experiment also revealed that oxygen consumption only increased above 24 °C, indicating that oxygen delivery becomes constrained beyond this threshold. This also provided a potential explanation for the limited behavioural changes observed at lower temperatures. In Chapter 4, I ran a long-term experiment simulating a marine heatwave and elevated suspended sediments, both individually and in combination. Results showed that increases in temperature resulted in energetic consequences. While oxygen consumption and survival were unaffected by treatments, fish in heatwave and the multistressor treatments experienced significant weight loss, demonstrating an energetic deficit where metabolic demands exceeded intake. Sedimentation alone did not cause weight loss, suggesting that temperature stress is the primary driver of reduced performance. These results indicate that while F. lapillum can adapt to fluctuating conditions, long or intense heat stress compromises energy use. During the gut content analysis conducted in Chapter 2, I found potential microplastics in F. lapillum gastrointestinal tracts. To further investigate microplastic accumulation in F. lapillum, in Chapter 5 I assessed the presence and identified the types of microplastic in F. lapillum gastrointestinal tracts, comparing between areas with different urbanisation levels. Microplastics were detected in every gastrointestinal tract analysed, and fragments were the most common morphotype found in both areas. F. lapillum from sites closer to the urban centre ingested higher microplastic loads. These findings indicate widespread microplastic contamination in Wellington’s coastal waters and suggest that urbanisation intensifies exposure. In order to try and predict some of the ecosystem-scale impacts of increasing suspended sediment concentrations, in Chapter 6 I develop a method to incorporate sedimentation effects into a multispecies size-spectrum model of the Tasman and Golden Bays (TBGB) ecosystem using the R package mizer. Using the observed effects of sedimentation on feeding efficiency from Chapter 4, I modified predator-prey encounter rates by reducing the search volume in the TBGB model to simulate sediment-driven reductions in predation success. Reductions in search volume altered predator prey interactions, leading to a reorganisation of trophic roles within the ecosystem. At the community level, the model predicted an overall decline in total biomass and productivity, accompanied by a redistribution of biomass among species groups. Demersal and benthic associated species exhibited greater tolerance to sedimentation, whereas the largest size classes experienced the strongest biomass declines, with potential consequences for spawning stocks and long term ecosystem resilience. The approach provides a novel method for incorporating SSC impacts in multispecies size-spectrum models, enabling researchers and managers to explore how sedimentation may alter species interactions, size structure, and community dynamics under future climate scenarios. Overall, the results of my thesis demonstrate that F. lapillum can exhibit phenotypic and behavioural plasticity in response to fluctuations in temperature and sedimentation. However, this common intertidal species fails to meet its metabolic demands under prolonged periods of elevated temperature, and demonstrates impaired feeding performance when faced with high levels of sedimentation. At the population level, temperature variability strongly shapes body condition, abundance, and diet. Moreover, the prevalence of microplastic ingestion underscores the impact of human-actions on New Zealand’s coastal fauna. Finally, the development of a sedimentation-integrated ecosystem model provides a powerful tool for advancing predictions of climate-driven change in coastal communities. Collectively, these findings highlight the vulnerability of intertidal fish to interacting climatic and anthropogenic stressors and emphasise the need to integrate physiological, behavioural, ecological, and modelling approaches to inform climate-resilient coastal management.