Benthic infauna of mangrove forests: dissolved oxygen and environmental settings determine their community composition and function
Mattone, Carlo (2016) Benthic infauna of mangrove forests: dissolved oxygen and environmental settings determine their community composition and function. PhD thesis, James Cook University.
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Abstract
Despite their importance in regulating many ecological processes the benthic infauna of mangrove forests are poorly understood. The anoxic nature of the sediments in which mangroves grow is likely to influence the dissolved oxygen (DO) dynamics of water in mangrove forests, and in turn influence the way benthic organisms utilize mangrove habitats. Despite this, little research has focused on understanding DO dynamics of the overlying water during tidal inundation and in the remnant pools formed after the tide ebbed and how this affects biological interactions. Therefore this study aimed to fill this gap and to assess the extent to which DO and environmental settings influence benthos composition and distribution in tropical Rhizophora stylosa mangrove forests. We assessed the DO patterns at various distances within an estuarine Rhizophora stylosa forest in tropical north Queensland over a total of 32 tidal cycles encompassing multiples seasons and tidal ranges. There were substantial fluctuations in DO, often varying from normoxic (100% saturation) to hypoxic (<10% saturation) within the same tidal cycle. A number of factors and their interactions influenced DO dynamics, in particular: tidal height, time of the day, tidal phase and distance from the forest edge. For instance, during spring nocturnal tide DO was relatively high during both flooding and ebbing phase, however during diurnal tide the water would become oversaturated in DO during the ebbing phase. On the other hand, during neap tide DO would exhibit a declining profile during both diurnal and nocturnal tides. Moreover during tidal disconnection the remnant pools within the forest quickly became anoxic, and low DO saturations are likely to affect how nekton and benthic organisms utilize mangrove forest. The trend for repeated declines to low DO levels correlates with a paucity of benthic taxa within estuarine R. stylosa forests, generally dominated by DO tolerant species such as sipunculids and capitellids. Furthermore, peracarids (i.e. amphipods, tanaids, mysids) were never found within the forests despite their occurrence on the adjacent intertidal flats. Because peracarids are a key component of juvenile fish diets, their absence within the forest is likely to limit foraging ground for juvenile nektons. Based on these results, we used the amphipod Melita spp. as a case study to test whether the DO fluctuations experienced within the mangrove forests were responsible for the lack of peracarids. Melita spp. was highly susceptible to low DO with less than 30% of the tested individuals surviving at 10% DO. Additionally, behavioural stress started to appear at around 20% DO. As the mangroves investigated experience DO lower than 5% saturation, it is plausible that Melita and other peracarids cannot use these estuarine R. stylosa forests due to the unsuitable DO conditions.
Building on these findings, we tested whether R. stylosa forests located in different environmental settings (e.g. non-estuarine) have differences in benthic communities. The comparisons involved two estuarine and two island R. stylosa forests located in tropical north Queensland in close proximity to one another. The results highlight distinct differences in benthic communities between the island and the estuarine R. stylosa forests. In particular the two forests were dominated by different polychaete families, with Opheliidae and Spionidae characterizing the island forests, and Glyceridae and Capitellidae characterizing the estuarine forests. Additionally, in contrast to the estuarine situation, peracarids were observed throughout the island forests. Such variability in dominant taxonomic groups is likely to affect food-web interactions as well as the function of the mangrove forest within the system. Limited DO sampling indicated overall higher DO levels at island sites compared to the estuarine conditions. These differences could be the result of higher wave energy that limits organic matter deposition within the island forests, leading to reduced DO depletion and different benthic communities. The importance that environmental settings have on determining infauna communities within R. stylosa forests could explain some of the variability in mangrove utilization by aquatic fauna between studies conducted in the Caribbean and the Indo-Pacific. Research in the Caribbean is often carried out in island-type mangroves, while in the Indo-Pacific the majority of studies have focused on estuarine mangroves. The results of this thesis suggest that DO dynamics, together with environmental settings, are important in determining fauna utilization of mangrove forests. Consequently, great caution should be taken when generalizing mangrove forest ecological functions.
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