Shelf Transport Pathways Adjacent to the East Australian Current Reveal Sources of Productivity for Coastal Reefs

The region where the East Australian Current (EAC) separates from the coast is dynamic and the shelf circulation is impacted by the interplay of the western boundary current and its eddy field with the coastal ocean. This interaction can drive upwelling, retention or export. Hence understanding the connection between offshore waters and the inner shelf is needed as it influences the productivity potential of valuable coastal rocky reefs. Near urban centres, artificial reefs enhance fishing opportunities in coastal waters, however these reefs are located without consideration of the productivity potential of adjacent waters. Here we identify three dominant modes of mesoscale circulation in the EAC separation region (~31.5−34.5°S); the ‘EAC mode’ which dominates the flow in the poleward direction, and two eddy modes, the ‘EAC eddy mode’ and the ‘Eddy dipole mode’, which are determined by the configuration of a cyclonic and anticyclonic eddy and the relationship with the separated EAC jet. We use a Lagrangian approach to reveal the transport pathways across the shelf to understand the impact of the mesoscale circulation modes and to explore the productivity potential of the coastal waters. We investigate the origin (position and depth) of the water that arrives at the inner-mid shelf over a 21-day period (the plankton productivity timescale). We show that the proportion of water that is upwelled from below the euphotic zone varies spatially, and with each mesoscale circulation mode. Additionally, shelf transport timescales and pathways are also impacted by the mesoscale circulation. The highest proportion of upwelling (70%) occurs upstream of 32.5°S, associated with the EAC jet separation, with vertical displacements of 70–120 m. From 33 to 33.5°S, water comes from offshore above the euphotic layer, and shelf transport timescales are longest. The region of highest retention over the inner shelf is immediately downstream of the EAC separation region. The position of the EAC jet and the location of the cyclonic eddy determines the variability in shelf-ocean interactions and the productivity of shelf waters. These results are useful for understanding productivity of temperate rocky reefs in general and specifically for fisheries enhancements along an increasingly urbanised coast

Driving the blue fleet: Temporal variability and drivers behind bluebottle (Physalia physalis) beachings off Sydney, Australia

Physalia physalis, the bluebottle in Australia, are colonial siphonophores that live at the surface of the ocean, mainly in tropical and subtropical waters. P. physalis are sometimes present in large swarms, and with tentacles capable of intense stings, they can negatively impact public health and commercial fisheries. P. physalis, which does not swim, is advected by ocean currents and winds acting on its gas-filled sail. While previous studies have attempted to model the drift of P. physalis, little is known about its sources, distribution, and the timing of its arrival to shore. In this study, we present a dataset with four years of daily P. physalis beachings and stings reports at three locations off Sydney's coast in Australia. We investigate the spatial and temporal variability of P. physalis presence (beachings and stings) in relation to different environmental parameters. This dataset shows a clear seasonal pattern where more P. physalis beachings occur in the Austral summer and less in winter. Cold ocean temperatures do not hinder the presence of P. physalis and the temperature seasonal cycle and that observed in P. physalis presence/absence time-series are out of phase by 3-4 months. We identify wind direction as the major driver of the temporal variability of P. physalis arrival to the shore, both at daily and seasonal time-scales. The differences observed between sites of the occurrence of beaching events is consistent with the geomorphology of the coastline which influences the frequency and direction of favorable wind conditions. We also show that rip currents, a physical mechanism occurring at the scale of the beach, can be a predictor of beaching events. This study is a first step towards understanding the dynamics of P. physalis transport and ultimately being able to predict its arrival to the coast and mitigating the number of people who experience painful stings and require medical help

Nitrate Sources, Supply, and Phytoplankton Growth in the Great Australian Bight: An Eulerian-Lagrangian Modeling Approach

The Great Australian Bight (GAB), a coastal sea bordered by the Pacific, Southern, and Indian Oceans, sustains one of the largest fisheries in Australia but the geographical origin of nutrients that maintain its productivity is not fully known. We use 12 years of modeled data from a coupled hydrodynamic and biogeochemical model and an Eulerian-Lagrangian approach to quantify nitrate supply to the GAB and the region between the GAB and the Subantarctic Australian Front (GAB-SAFn), identify phytoplankton growth within the GAB, and ascertain the source of nitrate that fuels it. We find that nitrate concentrations have a decorrelation timescale of ∼60 days; since most of the water from surrounding oceans takes longer than 60 days to reach the GAB, 23% and 75% of nitrate used by phytoplankton to grow are sourced within the GAB and from the GAB-SAFn, respectively. Thus, most of the nitrate is recycled locally. Although nitrate concentrations and fluxes into the GAB are greater below 100 m than above, 79% of the nitrate fueling phytoplankton growth is sourced from above 100 m. Our findings suggest that topographical uplift and stratification erosion are key mechanisms delivering nutrients from below the nutricline into the euphotic zone and triggering large phytoplankton growth. We find annual and semiannual periodicities in phytoplankton growth, peaking in the austral spring and autumn when the mixed layer deepens leading to a subsurface maximum of phytoplankton growth. This study highlights the importance of examining phytoplankton growth at depth and the utility of Lagrangian approaches

Six years of demography data for 11 reef coral species

Scleractinian corals are colonial animals with a range of life history strategies, making up diverse species assemblages that define coral reefs. We tagged and tracked approximately 30 colonies from each of 11 species during seven trips spanning six years (2009-2015) in order to measure their vital rates and competitive interactions on the reef crest at Trimodal Reef, Lizard Island, Australia. Pairs of species were chosen from five growth forms where one species of the pair was locally rare (R) and the other common (C). The sampled growth forms were massive [Goniastrea pectinata (R) and G. retiformis (C)], digitate [Acropora humilis (R) and A. cf. digitifera (C)], corymbose [A. millepora (R) and A. nasuta (C)], tabular [A. cytherea (R) and A. hyacinthus (C)] and arborescent [A. robusta (R) and A. intermedia (C)]. An extra corymbose species with intermediate abundance, A. spathulata was included when it became apparent that A. millepora was too rare on the reef crest, making the 11 species in total. The tagged colonies were visited each year in the weeks prior to spawning. During visits, two or more observers each took 2-3 photographs of each tagged colony from directly above and on the horizontal plane with a scale plate to track planar area. Dead or missing colonies were recorded and new colonies tagged in order to maintain approximately 30 colonies per species throughout the six years of the study. In addition to tracking tagged corals, 30 fragments were collected from neighboring untagged colonies of each species for counting numbers of eggs per polyp (fecundity); and fragments of untagged colonies were brought into the laboratory where spawned eggs were collected for biomass and energy measurements. We also conducted surveys at the study site to generate size structure data for each species in several of the years. Each tagged colony photograph was digitized by at least two people. Therefore, we could examine sources of error in planar area for both photographers and outliners. Competitive interactions were recorded for a subset of species by measuring the margins of tagged colony outlines interacting with neighboring corals. The study was abruptly ended by Tropical Cyclone Nathan (Category 4) that killed all but nine of the over 300 tagged colonies in early 2015. Nonetheless, these data will be of use to other researchers interested in coral demography and coexistence, functional ecology, and parametrizing population, community and ecosystem models. The data set is not copyright restricted, and users should cite this paper when using the data.Publisher PDFPeer reviewe

Modelling physical and biological drivers of larval retention in reef systems

Larval dispersal has major effects on ecological and evolutionary processes. Thus, a thorough understanding of mechanisms driving larval transport, and estimates of larval retention, are necessary to inform the conservation and management of marine biodiversity. The overall aim of this thesis was to investigate physical and biological factors driving larval transport in coral reef systems, with particular emphasis on the effect of lee‐reef eddies, swimming behaviour, and their interaction.\ud \ud In the second chapter, I quantified the extent to which lee‐reef eddies with different dynamics influence retention of passive larvae close to reefs. Simulations of particle transport at individual reefs with idealized shapes, under the influence of unidirectional and tidal flows, were conducted with the Sparse Hydrodynamic Ocean Code (SHOC), a three dimensional hydrodynamic model, using a reef‐scale spatial resolution. I then tested how accurately the Island Wake Parameter (I), a dimensionless number that indicates the degree of turbulence of flow past obstacles under stationary flows in shallow waters, characterized the qualitative nature of the simulated flow past reefs. Theoretical considerations suggest that I~1 implies the formation of stable eddies that remain attached downstream of the reef, and I>10 implies the formation of unstable eddies that detach and dissipate or are advected downstream. I found that the Island Wake Parameter captured adequately the qualitative nature of the flow past idealized reefs. Unidirectional flows induced the formation of stable eddies and were associated with I~1, while tidal flows induced the detachment and downstream advection of eddies and were associated with I>10. These eddies provoked the recirculation of particles, prolonging retention times. Thus, stable eddies (small I values) induced long retention times, and unstable eddies (large I values) induced short retention times. Indeed, a nonlinear regression indicated that the Island Wake Parameter explained 81‐92% of the variability in retention time among idealized reefs across the range of flow regimes considered. This suggests that the Island Wake Parameter is a useful predictor of the formation and duration of eddies past reefs with idealized shapes under unidirectional and tidal flows, and that eddies and their life‐spans are key determinants of the retention of passive particles close to reefs.\ud \ud The findings above suggest that the Island Wake Parameter may be useful for approximating retention of particles close to reefs, when well‐calibrated, reef‐scale circulation models are not unavailable. To investigate this possibility further, I aimed, first, to examine how well the Island Wake Parameter characterizes the flow past real reefs under nonstationary flows, and second, under these more realistic circulation regimes, to determine effect of eddies on retention of passive larvae, and test the robustness of the relationship between the Island Wake Parameter and mean retention times. To achieve these objectives, in Chapter 3, I implemented SHOC with a reef‐scale spatial resolution (~300 m) within the central Great Barrier Reef (GBR), encompassing 14 middle and 6 outer shelf reefs with various shapes (crescentic, lagoonal, planar and patchy), ranging from 1.9 to 27.5 km2 in size. Comparison of the model outcomes against observed time series of temperature, sea level and currents through correlation, principal components, and spectral analyses, showed that the model reproduces the dynamics in the region adequately. In particular, it characterizes well the formation of eddies downstream of reefs, and the upwelling events associated with these eddies.\ud \ud Having validated the central GBR model, I turned in Chapter 4 to simulating passive larval transport over two spawning events, and to quantifying retention time at individual reefs. The Island Wake Parameter proved successful at discerning between the presence and absence of eddies. In turn, the presence of eddies, and their duration, strongly influenced larval retention: the longest retention times occurred at reefs where eddies were long‐lived, and the shortest retention times when eddies did not form at all. Finally, a common functional relationship characterized how mean retention time depends on the Island Wake Parameter, both for idealized reefs under simplified flows (from Chapter 2), and for reefs along the central GBR under realistic circulation. These results indicate, first, that lee‐reef eddies and their dynamics are accurately depicted by the Island Wake Parameter, and second, that lee‐reef eddies are key drivers of passive larval transport close to reefs. Because the Island Wake Parameter is a simple function of upstream flow velocity, reef geometry, and vertical diffusion, these findings suggest that first‐order estimates of larval retention may be obtained from relatively coarse‐scale characteristics of the flow, and basic features of reef geomorphology. Such approximations may be a valuable tool for modelling meta‐population dynamics over large spatial scales, where explicitly characterizing fine‐scale flows around reefs would require extensive computational resources and model calibration.\ud \ud The results of Chapters 2 and 4 indicate that the potential of lee‐reef eddies to retain particles is limited by eddy life‐span. Stable eddies can facilitate self‐recruitment of species whose larvae are passive, or weak swimmers, for a few days after release. However, mean currents along the central GBR are intense and provoke unstable eddies that only retain larvae for periods of less than a day. This suggests that other mechanisms, such as swimming behaviour, might play an important role in larval retention and dispersal patterns of species whose larvae take weeks or months to develop, such as reef fishes.\ud \ud Consequently, the objective of the fifth chapter was to quantify the extent to which horizontal swimming behaviour of larvae that exploit lee reef eddies can influence retention close to reefs. I incorporated larval behaviour within SHOC's Lagrangian algorithm to simulate the transport of larvae that swim towards regions where lee reef eddies form. The implemented swimming speed of larvae increased with age according to an empirical relationship for Lutjanids. Simulations were conducted at idealized reefs under alternative circulation regimes that produced stable eddies, unstable eddies, and no eddies. Larvae were assumed to be advected into the reef's vicinity at different stages of their development, and the proportion of retained larvae at the end of the pelagic larval duration was quantified for each larval stage and circulation regime. Depletion of energy reserves by swimming was also considered, based on published data. Results indicate that the potential for swimming behaviour to increase retention close to reefs is highly sensitive to the extent to which larval feeding is sufficient to replenish energy reserves, and on the presence and duration of eddies. If food sources are sufficiently available throughout the planktonic stage to replenish energy expended during swimming, then larval swimming behavior suffices to enhance retention close to reefs and facilitate self‐recruitment. However, if food is scarce, the presence of favourable circulation structures, such as eddies with long life‐spans, or vertical migration combined with currents flowing in varying directions with depth, are necessary for self-recruitment, and crucial to enhance the retention of late‐stage pelagic larvae that arrives close to reefs

Driving the blue fleet: Temporal variability and drivers behind bluebottle (Physalia physalis) beachings off Sydney, Australia

Physalia physalis , the bluebottle in Australia, are colonial siphonophores that live at the surface of the ocean, mainly in tropical and subtropical waters. P. physalis are sometimes present in large swarms, and with tentacles capable of intense stings, they can negatively impact public health and commercial fisheries. P. physalis , which does not swim, is advected by ocean currents and winds acting on its gas-filled sail. While previous studies have attempted to model the drift of P. physalis , little is known about its sources, distribution, and the timing of its arrival to shore. In this study, we present a dataset with four years of daily P. physalis beachings and stings reports at three locations off Sydney’s coast in Australia. We investigate the spatial and temporal variability of P. physalis presence (beachings and stings) in relation to different environmental parameters. This dataset shows a clear seasonal pattern where more P. physalis beachings occur in the Austral summer and less in winter. Cold ocean temperatures do not hinder the presence of P. physalis and the temperature seasonal cycle and that observed in P. physalis presence/absence time-series are out of phase by 3-4 months. We identify wind direction as the major driver of the temporal variability of P. physalis arrival to the shore, both at daily and seasonal time-scales. The differences observed between sites of the occurrence of beaching events is consistent with the geomorphology of the coastline which influences the frequency and direction of favorable wind conditions. We also show that rip currents, a physical mechanism occurring at the scale of the beach, can be a predictor of beaching events. This study is a first step towards understanding the dynamics of P. physalis transport and ultimately being able to predict its arrival to the coast and mitigating the number of people who experience painful stings and require medical help

Driving the blue fleet: Temporal variability and drivers behind bluebottle (Physalia physalis) beachings off Sydney, Australia

Physalia physalis , the bluebottle in Australia, are colonial siphonophores that live at the surface of the ocean, mainly in tropical and subtropical waters. P. physalis are sometimes present in large swarms, and with tentacles capable of intense stings, they can negatively impact public health and commercial fisheries. P. physalis , which does not swim, is advected by ocean currents and winds acting on its gas-filled sail. While previous studies have attempted to model the drift of P. physalis , little is known about its sources, distribution, and the timing of its arrival to shore. In this study, we present a dataset with four years of daily P. physalis beachings and stings reports at three locations off Sydney’s coast in Australia. We investigate the spatial and temporal variability of P. physalis presence (beachings and stings) in relation to different environmental parameters. This dataset shows a clear seasonal pattern where more P. physalis beachings occur in the Austral summer and less in winter. Cold ocean temperatures do not hinder the presence of P. physalis and the temperature seasonal cycle and that observed in P. physalis presence/absence time-series are out of phase by 3-4 months. We identify wind direction as the major driver of the temporal variability of P. physalis arrival to the shore, both at daily and seasonal time-scales. The differences observed between sites of the occurrence of beaching events is consistent with the geomorphology of the coastline which influences the frequency and direction of favorable wind conditions. We also show that rip currents, a physical mechanism occurring at the scale of the beach, can be a predictor of beaching events. This study is a first step towards understanding the dynamics of P. physalis transport and ultimately being able to predict its arrival to the coast and mitigating the number of people who experience painful stings and require medical help

Mesoscale circulation determines broad spatio-temporal settlement patterns of lobster.

The influence of physical oceanographic processes on the dispersal of larvae is critical for understanding the ecology of species and for anticipating settlement into fisheries to aid long-term sustainable harvest. This study examines the mechanisms by which ocean currents shape larval dispersal and supply to the continental shelf-break, and the extent to which circulation determines settlement patterns using Sagmariasus verreauxi (Eastern Rock Lobster, ERL) as a model species. Despite the large range of factors that can impact larval dispersal, we show that within a Western Boundary Current system, mesoscale circulation explains broad spatio-temporal patterns of observed settlement including inter-annual and decadal variability along 500 km of coastline. To discern links between ocean circulation and settlement, we correlate a unique 21- year dataset of observed lobster settlement (i.e., early juvenile & pueruli abundance), with simulated larval settlement. Simulations use outputs of an eddy-resolving, data-assimilated, hydrodynamic model, incorporating ERL spawning strategy and larval duration. The latitude where the East Australian Current (EAC) deflects east and separates from the continent determines the limit between regions of low and high ERL settlement. We found that years with a persistent EAC flow have low settlement while years when mesoscale eddies prevail have high settlement; in fact, mesoscale eddies facilitate the transport of larvae to the continental shelf-break from offshore. Proxies for settlement based on circulation features observed with satellites could therefore be useful in predicting broadscale patterns of settlement orders of magnitudes to guide harvest limits

Strengthened currents override the effect of warming on lobster larval dispersal and survival

Human-induced climate change is projected to increase ocean temperature and modify circulation patterns, with potential widespread implications for the transport and survival of planktonic larvae of marine organisms. Circulation affects the dispersal of larvae, whereas temperature impacts larval development and survival. However, the combined effect of changes in circulation and temperature on larval dispersal and survival has rarely been studied in a future climate scenario. Such understanding is crucial to predict future species distributions, anticipate ecosystem shifts and design effective management strategies. We simulate contemporary (1990s) and future (2060s) dispersal of lobster larvae using an eddy-resolving ocean model in south-eastern Australia, a region of rapid ocean warming. Here we show that the effects of changes in circulation and temperature can counter each other: ocean warming favours the survival of lobster larvae, whereas a strengthened western boundary current diminishes the supply of larvae to the coast by restricting cross-current larval dispersal. Furthermore, we find that changes in circulation have a stronger effect on connectivity patterns of lobster larvae along south-eastern Australia than ocean warming in the future climate so that the supply of larvae to the coast reduces by ~4% and the settlement peak shifts poleward by ~270 km in the model simulation. Thus, ocean circulation may be one of the dominant factors contributing to climate-induced changes of species ranges