Dissolved oxygen as a constraint for the structure of mangrove fish assemblages and their patterns of mangrove utilisation: a comparison between natural and disturbed mangrove ecosystems through the Australian and Caledonian coastlines

Mangroves are commonly recognised as important fish habitats. However, studies that have quantified their utilisation by fish often found contrasting results, supporting the contention that mangrove forests provide heterogeneous values. Mangroves are dynamic, fluctuating and challenging environments consequently inducing variability in mangrove fish assemblages. Our understanding on the factors contributing to this variability is still limited, but essential to manage and conserve these valuable and threatened habitats.

Tide has been identified as one of the key factors to explain variations in mangroves utilisation. Tide is responsible for many physical changes such as water depth, current, connectivity, but also chemical changes such as dissolved oxygen (DO), salinity, temperature and pH. Considering the numerous changes driven by tide, it is difficult to quantify their specific influence on fish assemblages, and as a result, their role and relative importance is not fully understood. The objective of this thesis was to provide new understanding on temporal and spatial variability in the use of intertidal mangrove habitats by fish in response to fluctuating environmental factors. I identified that the two major challenges that fauna face while using intertidal mangrove habitats are the total drainage at low tide inducing a risk of stranding, and hypoxic conditions that can lead to physiological stress and death. Consequently, the importance of depth, hypoxia, but also the nature of the mangrove habitat were principally investigated as evidence shows that they could be key factors determining mangrove utilisation. 

To begin understanding how hypoxia can limit the utilisation of intertidal mangrove habitats, DO and depth loggers were deployed over several days and tidal cycles across a mangrove wetland innorth-east Australia (Chapter 2). I characterised the DO dynamics at a short-time scale (tidal and diel) and a fine spatial scale and I investigated the effects of tidal factors on DO fluctuations. I also determined potential risks from exposure to hypoxic events for fish occupying this wetland. Results showed that DO daily reached harmful and occasionally lethal levels, and therefore had the potential to temporarily limit the utilisation of this wetland by fish. I also identified that DO closely followed tidal fluctuations. 

To extend our knowledge on variations in mangrove fish assemblages in relation to environmental fluctuations, I deployed underwater video cameras coupled with multi- parameter loggers (DO, depth, salinity, temperature). I sampled on the edge of, and 5 m inside (in-forest), an intertidal mangrove forest, New Caledonia, to identify whether these two major components of mangrove forests were used differently. Chapter 3 showed that fish assemblages on the mangrove edge and in-forest were highly distinct, with most species observed on the edge and few species entering in-forest. Fish assemblages also varied temporally across tide, most likely explained by species-specific tidal migrations, highlighting that fish responded to one or several environmental factors fluctuating with tide. The most evident factor varying with tide, and that can trigger tidal migrations, is water depth as the mangrove forest becomes temporarily unavailable to fish. However, chapter 2 showed that DO and depth are highly correlated, and that DO can reach harmful levels in intertidal mangrove habitats. As depth and DO are two potentially limiting factors that vary across tide, I attempted to disentangle their confounding effects in causing fish tidal variations. Depth was statistically identified as the factor with the highest predictive power in explaining variations in fish assemblages, with species preferentially accessing mangrove habitats at different depth ranges. However, DO also varied with tide in this mangrove forest, reaching harmful levels especially early in the morning at low tide (Chapter 4). DO was also statistically identified as a key factor explaining fish tidal variations, therefore, I further investigated how fish responded to DO fluctuations. I found that fish displayed species-specific variations in response to DO, apparently explained by differential behavioural avoidance thresholds. Indeed, three distinct groups of species were identified based on distinct preferences for DO with species recorded from 30 to 110 % saturation, species recorded from 50-110 % saturation, and species recorded from 70 to 110 % saturation. Thus, I hypothesised that species-specific responses to DO could be explained by differential tolerances to hypoxia.

I used the results from chapter 3 to design laboratory experiments and test for this hypothesis (Chapter 5). I selected four species displaying distinct preferences for DO and I determined their physiological tolerances to hypoxia using intermittent-flow respirometry during which I decreased DO in accordance to what I observed in the natural environment. Results showed a causality between species physiological tolerance to hypoxia and mangrove habitat utilisation in response to DO. They suggest that species able to withstand low DO use mangrove habitats more extensively than more sensitive species. Consequently, it is probable that being highly tolerant to hypoxia is a required pre-condition to thrive in these extreme environments, which could help explaining the little use of some intertidal mangrove forests. 

The present research highlights that mangrove forests utilisation is highly variable over space and time. Their utilisation varies spatially depending on which components of the mangrove forest are studied such as the edge or the in-forest. Intertidal mangroves utilisation also varies temporally as tide creates many changes that can constraint fish. This study especially provides important knowledge on how DO can impact fish communities by favouring highly tolerant species to hypoxia, and on the other hand exclude more sensitive species. This new information can help explain the contrasting results found in the literature concerning the utilisation of mangrove forests. For instance, the higher contribution of coral reef fish to the mangrove fish assemblages in the Caribbean compared to other regions of the Indo-Pacific could be due to a lower hypoxia tolerance among coral reef fish compared to intertidal mangrove species. This study shows that fish using mangroves can sometimes be living on the edge of oxygen limitations. In the view of further predicted human-caused oxygen decrease, it is crucial to address the gaps of knowledge concerning DO dynamics in mangrove habitats, and its impacts on fish populations. This can assist managers and policy makers to establish appropriate management and conservation plans to ensure the sustainability of mangrove habitats.

Topic: Marine and fisheries research
Subfield: Global and Planetary Change
Field: Environmental Science
Domain: Physical Sciences