Bounded and unbounded boundaries – Untangling mechanisms for estuarine-marine ecological connectivity: Scales of m to 10,000 km – A review
Graphical abstract
Introduction
Are estuaries and coastal waters unbounded systems? Are they connected with each other? If so, what are the temporal and spatial scales of self-recruitment and connectivity? Answering these questions is essential to quantify estuarine ecosystem connectivity and organismal self-recruitment, and required to understand habitats, biotopes and assemblages (collectively comprising ‘seascapes’) and quantify their ecological structure and resilience (Cowen et al., 2000, Cowen et al., 2006, Cowen et al., 2007, Swearer et al., 2002, Ray, 2005, Gawarkiewicz et al., 2007, Mumby and Hastings, 2008, Sheaves, 2009, Sheaves, 2016, Barbosa and Chicharo, 2011, Chicharo et al., 2012, Robins et al., 2013, Dias et al., 2014).
Various species within a seascape release plant fragments, eggs and sperm, spores, propagules, polyps, larvae, hatchlings and juveniles into the water column and these may be waterborne in their early life stages and therefore they are advected, diffused and dispersed by the water currents. These also have a behaviour, which can be passive or active, such as choosing to float or to sink, or a behavioural selection of a preferred depth, directional swimming, and choosing when and where to settle. This behaviour, coupled with the vagaries of the water currents, determines the degree of self-recruitment and connectivity between the spawning grounds and the settlement grounds and this determine the seascape (e.g. Sheaves, 2016). This physical control creates the fundamental niches of the water column and seabed thus allowing colonisation by organisms to create ecological structure, interactions between organisms to create ecological functioning and feedback loops whereby the organisms influence the niche state (Wolanski and Elliott, 2015). The dispersal of planktonic phases of benthic organisms and early life stages of fishes creates the fundamental property of connectivity between each part of the whole aquatic ecosystem (the river catchment, the estuaries, the coasts and the open seas). This connectivity ensures the appropriate ecological structure and functioning (Elliott and Whitfield, 2011, Whitfield and Elliott, 2011, Basset et al., 2013). Furthermore, the management of any one part of this aquatic continuum requires an understanding of the links with all other parts.
Physical oceanographers have long attempted to explain the patterns of self-recruitment and the connectivity of marine populations that until recently were commonly assumed to be moving passively in the water column with no behaviour. As detailed below, in most cases these physical oceanographic studies did not explain satisfactorily the biological and genetic observations, and this mismatch between observations and predictions suggested that additional processes than just oceanography must be taken into account for the models to be realistic (Levin, 2006, Becker et al., 2007, Gawarkiewicz et al., 2007, Gaspar et al., 2012, Wolanski and Elliott, 2015, Pfaff et al., 2015). To overcome problems of interpretation in the literature, here behaviour is regarded as being passive or active, where even if an organism is not moving (actively) by its appendages, its behaviour is (passively) created by its body morphology and anatomy (e.g. dimensions, presence of oil droplets for buoyancy, support appendages and aerofoils).
This paper focuses on the combined importance for self-recruitment and connectivity of behaviour and physical, chemical and biological oceanographic processes. The latter include chemical barriers, the pelagic larval duration (PLD), and the swimming strategy in response to a number of cues. It shows that a more realistic understanding of the connectivity and self-recruitment results when the active and passive behaviour of the plant fragments, eggs and sperm, spores, propagules, larvae, hatchlings and juveniles is taken into account. This paper presents certain specific case studies that successfully linked physics, chemistry and biology to quantify or at least explain quantitatively the processes of self-recruitment. These case studies are each briefly summarised to highlight the dominant behaviour and the oceanographic, chemical or biological processes that are shaping the resultant biotope. The case studies are ordered according to the apparent increasing importance of behaviour and scale in determining self-recruitment: jellyfish in marine lakes, mangrove propagules, corals in acidified bays, seagrass, oysters and mussels, prawns and some estuarine fish larvae, and finally sea turtles. All these studies are restricted to one generation. However, for some species the self-recruitment occurs over two or three generations such as for the copepod Calanus finmarchius in the North Sea and the ornate spiny lobster Panulirus ornatus from Australia-Papua New Guinea to Vietnam. These are also described.
These case studies demonstrate the combined importance of behaviour and oceanography in controlling the self-recruitment and connectivity of estuarine and coastal fauna and flora. Estuarine and coastal systems are found to be simultaneously bounded and unbounded depending on the sites and the species considered. The analysis here aims to show that for the various species whose self-recruitment process is described in this paper, the more a system is physically unbounded for a species, the more important and the more elaborate is the behaviour of that species in order to self-recruit. This suggests that future advances in our understanding of the self-recruitment and connectivity of estuarine and coastal fauna and flora requires a better integration of physical oceanography field and model studies with laboratory and field studies of the behaviour of the organisms, microchemical tagging studies using natural and artificial markers, direct observations of trajectories, and population genetics studies.
Section snippets
Physico-chemical barriers: Palau’s jellyfish lake
Many estuaries worldwide suffer from low dissolved oxygen (DO) concentration which, at <4 mg l−1, causes stress in migrating fish, a barrier to migration and thus inhibiting ecological connectivity in the sea-estuarine-catchment continuum (Elliott and Hemmingway, 2002, McLusky and Elliott, 2004, Wolanski and Elliott, 2015, Tétard et al., 2016). These effects are seasonal in most estuaries that have a period of high river flow (e.g. from spring runoff or cyclone floods) that flushes out the
A synthesis
Estuarine and coastal physical oceanographers have made a significant contribution to understand and quantify the water currents and the diffusion processes that help control the transport of propagules and larvae, assuming that they were passive. It was assumed that this would explain the self-recruitment and the connectivity of marine populations. However, when compared with results from tagging and population genetic studies, in many cases these oceanographic models did not satisfactorily
Acknowledgments
It is a pleasure to thank Takashi Asaeda at Saitama University in Japan, Hoc Tan Dao at the Oceanography Institute in Vietnam, Michael Dawson at the University of California Merced in the USA, Eric Deleersnijder and Jonathan Lambrechts at the Université catholique de Louvain in Belgium, Michael Elliott at the University of Hull in the UK, Keita Furukawa at the Ocean Policy Research Foundation in Japan, Yimnang Golbuu at the Palau International Coral Reef Centre in Palau, Alana Grech at
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