ALBERT

Description: A two-week field experiment investigated the hydrodynamics of a strongly tidally-forced tropical intertidal reef platform in the Kimberley region of northwestern Australia, where the spring tidal range exceeds 8 m. At this site, the flat and wide (∼1.4 km) reef platform is located slightly above mean sea level, such that during low tide the offshore water level can fall 4 m below the platform. While the reef always remained submerged over each tidal cycle, there were dramatic asymmetries in both the water levels and velocities on the reef, i.e., the flood duration lasted only ∼2 hr versus ∼10 hr for the ebb. These dynamics were investigated using a one-dimensional numerical model (SWASH) to solve the nonlinear shallow water equations with rapid (sub- to super-critical) flow transitions. The numerical model revealed that as water drains off the reef, a critical flow point was established near the reef edge prior to the water discharging down the steep forereef. Despite this hydraulic control, bottom friction on the reef was still found to make a far greater contribution to elevating water levels on the reef platform and keeping it submerged over each tidal cycle. Finally, a simple analytical model more broadly shows how water levels on intertidal reef platforms functionally depend on properties of reef morphology, bottom roughness, and tidal conditions, revealing a set of parameters (a reef draining time-scale and friction parameter) that can be used to quantify how the water depth will fall on a reef during ebb tide. This article is protected by copyright. All rights reserved.

Description: [1]  The shelf circulation off Ningaloo Reef near the North West Cape of Western Australia is driven by complex interactions between the southward flowing Leeuwin Current and wind-driven currents that episodically reverse the coastal flow towards the north. The presence of these northward (equatorward) wind-driven currents is thought to make this section of coast one of the few locations along Western Australia to experience periodic coastal upwelling. We used a combination of field observations and numerical modeling to investigate this summer circulation and upwelling dynamics along Ningaloo Reef. We analyzed current and temperature profiles from moorings at four sites across the shelf and used two Regional Ocean Modeling System (ROMS) sub-models: (1) a coarser model of northwestern Australia forced by a global ocean model and (2) a nested fine-scale model of the Ningaloo region. This nesting significantly improved model skill as it included the offshore mesoscale dynamics that strongly influenced the shelf circulation off Ningaloo. The field observations revealed several northward flow reversals, accompanied by cooling of the coastal waters adjacent to Ningaloo, which were associated with strong northward wind events. Analysis of the coastal heat budget revealed that cooling events were primarily driven by upwelling, whereas warming of coastal waters during relaxation events resulted mostly from along-shelf advection of warm water from the north. Due to the combined effects of its relatively steep (~1/50 slope) shelf and strong summer stratification, upwelled water was sourced from the interior of the water column, likely influencing the sources and fluxes of nutrients to Ningaloo Reef.

Description: Rates of net production, net calcification, and nutrient uptake were measured in a coral-dominated reef flat community on Ningaloo Reef in northwestern Australia under seasonally minimum and maximum light levels. Daily integrated light decreased twofold while water temperatures remained relatively constant increasing by only 1°C on average from summer to winter. Rates of daily community gross primary production (GPP) were only 33% ± 9% higher in summer than in winter (1400 ± 70 versus 1050 ± 60 mmol C m−2 d−1), far less than the twofold seasonal changes reported for most shallow reef communities. Rates of daily community net calcification (Gnet) were not significantly different between seasons (190 ± 40 mmol CaCO3 m−2 d−1 in summer versus 200 ± 10 mmol CaCO3 m−2 d−1 in winter). The average rate of total nitrogen uptake (dissolved + particulate) was also not significantly different between summer and winter (8.3 ± 3.8 versus 6.6 ± 3.4 mmol N m−2 d−1, respectively), despite evidence of sporadically high nitrate uptake in both seasons. In summer, rates of hourly net calcification (gnet) were linearly correlated with diurnal changes in net production, pH, and aragonite saturation state (Ωar); and were mostly correlated with light except at mid-day under heavy cloud cover. However, in winter, gnet was independent of diurnal changes in light, net production, pH, and Ωar indicating that the reef flat community had possibly reached a threshold above which rates of net calcification were insensitive to diurnal changes in their environment.

Description: Fish have been shown to produce high (10 to 48 mol %) magnesium calcite as part of the physiological mechanisms responsible for maintaining salt and water balance. The importance of this source to the marine carbon cycle is only now being considered. In this paper, we report the first measurements of the solubility of this CaCO3 in seawater. The resulting solubility (pK*sp = 5.89 ± 0.09) is approximately two times higher than aragonite and similar to the high magnesium calcite generated on the Bahamas Banks (pK*sp = 5.90). This high solubility of fish-produced CaCO3 is a result of the high magnesium content and not a product of micro-environments created by microbial activity. This material is soluble in near surface waters, contributing to the input of carbonate to surface ocean waters, and may at least partially explain the observed increase in total alkalinity above the aragonite saturation horizon.

Description: The role of waves, tide, and wind on the circulation of a fringing reef system was investigated using data collected during a 6 week field experiment in a section of Ningaloo Reef off Western Australia. The high correlation observed between current velocities and wave height throughout the system revealed the dominant role wave breaking plays in driving the overall reef-lagoon circulation, whereas the modulation of the currents at tidal frequencies suggested that the wave-driven currents responded to tidal variations in the mean water level over the reef. The influence of the various forcing mechanisms on the current field was investigated for both high- and low-frequency bands. Wave breaking was found to be the dominant forcing mechanism for the low-frequency (subtidal) currents, with the subtidal flow pattern consisting of a cross-reef flow over the reef, alongshore flow in the lagoon, and water exiting back to the ocean through the main channel. The tides controlled the high-frequency current variability via two mechanisms: one associated with the ebb-flood cycle of the tides and the second associated with tidal modulations of the wave-driven currents. Wind-forcing and buoyancy effects were both found to be negligible in driving the circulation and flushing of the system during the observation period. Flushing time scale estimates varied from as low as 2 h to more than a day for the wide range of observed incident wave heights. The results suggest that the circulation of Ningaloo Reef will be strongly influenced by even a small mean sea level rise.

Description: The circulation and temperature variability on the inner shelf near the North West Cape of Australia off Ningaloo Reef was investigated using field data obtained from two moorings deployed from 2004 to 2009. The results revealed that alongshore currents on the inner shelf were, on average, only weakly influenced by the offshore poleward (southward) Leeuwin Current flow, i.e., monthly averaged alongshore current velocities were ∼0.1 m s−1 or less. The presence of a consistent summer-time wind-driven equatorward (northward) counter flow on the inner-shelf (referred to in the literature as the Ningaloo Current) was not observed. Instead, the shelf waters were strongly influenced year-round by episodic subtidal current fluctuations (time scale 1–2 weeks) that were driven by local wind-forcing. Analysis of the current profiles showed that periods of strong equatorward winds were able to overcome the dominant poleward pressure gradient in the region, leading to upwelling on the inner-shelf. Contrary to prior belief, these events were not limited to summer periods. The forcing provided by these periodic wind events and the associated alongshore flows can explain much of the observed temperature variability (with timescales 〈 1 month) that influences Ningaloo Reef.

Description: Six ice beacons deployed in the Beaufort Sea during August 2011 tracked the anomalous export of multiyear sea ice from the Chukchi Sea through the Bering Strait to the Bering Sea between November 2011 and May 2012. These are the first observations in 34 years of ice beacon export through the Bering Strait. Using 34 years of passive microwave derived ice motion fields we find that during 2011-2012 southward ice motion in the Chukchi Sea persisted for a record six of seven months and that sea ice speeds were significantly faster than the long term mean. The combination of increased ice speeds and reduced likelihood of ice arch development through the strait culminated in the record export of 13.5 x 10 3 km 2 of sea ice through the Bering Strait. Monthly sea level pressure fields, dominated by an Aleutian Low and Siberian High, show anomalies in December and January played a role in initiating this event and forced multiyear ice into the southern Chukchi Sea. However these variations were small and typical of this area, yet we find no evidence of a similar export event in the last 34 years even though the forcing was similar to the climatology. This leads us to attribute this event to a change in the responsiveness of the Arctic ice pack to typical forcing mechanisms.

Description: Two marine heat wave events along Western Australia (WA) during the alternate austral summer periods of 2010/11 and 2012/13, both linked to strong La Niña conditions, severely impacted marine ecosystems over more than 12 degrees of latitude, which included the unprecedented bleaching of many coral reefs. Although these two heat waves were forced by similar large-scale climate drivers, the warming patterns differed substantially between events. The central coast of WA (south of 22 ° S) experienced greater warming in 2010/11, whereas the northwestern coast of WA experienced greater warming in 2012/13. To investigate how oceanic and atmospheric heat exchange processes drove these different spatial patterns, an analysis of the ocean heat budget was conducted by integrating remote sensing observations, in situ mooring data, and a high resolution (∼1 km) ocean circulation model (Regional Ocean Modeling System). The results revealed substantial spatial differences in the relative contributions made by heat advection and air-sea heat exchange between the two heat wave events. During 2010/11, anomalous warming driven by heat advection was present throughout the region, but was much stronger south of 22 ° S where the poleward-flowing Leeuwin Current strengthens. During 2012/13, air-sea heat exchange had a much more positive (warming) influence on sea surface temperatures (especially in the northwest), and when combined with a more positive contribution of heat advection in the north, this can explain the regional differences in warming between these two La Niña-associated marine heat wave events.

Description: The presence of large bottom roughness, such as that formed by benthic organisms on coral reef flats, has important implications for the size, concentration, and transport of suspended sediment in coastal environments. A three-week field study was conducted in approximately 1.5 m water depth on the reef flat at Ningaloo Reef, Western Australia, to quantify the cross-reef hydrodynamics and suspended sediment dynamics over the large bottom roughness (∼20–40 cm) at the site. A logarithmic mean current profile consistently developed above the height of the roughness; however, the flow was substantially reduced below the height of the roughness (canopy region). Shear velocities inferred from the logarithmic profile and Reynolds stresses measured at the top of the roughness, which are traditionally used in predictive sediment transport formulations, were similar but much larger than that required to suspend the relatively coarse sediment present at the bed. Importantly, these stresses did not represent the stresses imparted on the sediment measured in suspension and are therefore not relevant to the description of suspended sediment transport in systems with large bottom roughness. Estimates of the bed shear stresses that accounted for the reduced near-bed flow in the presence of large roughness vastly improved the relationship between the predicted and observed grain sizes that were in suspension. Thus the impact of roughness, not only on the overlying flow but also on bed stresses, must be accounted for to accurately estimate suspended sediment transport in regions with large bottom roughness, a common feature of many shallow coastal ecosystems. This article is protected by copyright. All rights reserved.