Scarla Weeks - Academia.edu (original) (raw)
Papers by Scarla Weeks
Large tropical and subtropical marine animals must meet their energetic requirements in a largely... more Large tropical and subtropical marine animals must meet their energetic requirements in a largely oligotrophic environment. Many planktivorous elasmobranchs, whose thermal ecolo-gies prevent foraging in nutrient-rich polar waters, aggregate seasonally at predictable locations throughout tropical oceans where they are observed feeding. Here we investigate the foraging and oceanographic environment around Lady Elliot Island, a known aggregation site for reef manta rays Manta alfredi in the southern Great Barrier Reef. The foraging behaviour of reef manta rays was analysed in relation to zooplankton populations and local oceanography, and compared to long-term sighting records of reef manta rays from the dive operator on the island. Reef manta rays fed at Lady Elliot Island when zooplankton biomass and abundance were significantly higher than other times. The critical prey density threshold that triggered feeding was 11.2 mg m-3 while zooplankton size had no significant effect on feeding. The community composition and size structure of the zooplankton was similar when reef manta rays were feeding or not, with only the density of zooplankton changing. Higher zooplankton biomass was observed prior to low tide, and long-term (~5 years) sighting data confirmed that more reef manta rays are also observed feeding during this tidal phase than other times. This is the first study to examine prey availability at an aggregation site for reef manta rays and it indicates that they feed in locations and at times of higher zoo-plankton biomass.
Coral reefs are in decline worldwide and monitoring activities are important for assessing the im... more Coral reefs are in decline worldwide and monitoring activities are important for assessing the impact of disturbance on reefs and tracking subsequent recovery or decline. Monitoring by field surveys provides accurate data but at highly localised scales and so is not cost-effective for reef scale monitoring at frequent time points. Remote sensing from satellites is an alternative and complementary approach. While remote sensing cannot provide the level of detail and accuracy at a single point than a field survey, the statistical power for inferring large scale patterns benefits in having complete areal coverage. This review considers the state of the art of coral reef remote sensing for the diverse range of objectives relevant for management, ranging from the composition of the reef: physical extent, benthic cover, bathymetry, rugosity; to environmental parameters: sea surface temperature, exposure, light, carbonate chemistry. In addition to updating previous reviews, here we also consider the capability to go beyond basic maps of habitats or environmental variables, to discuss concepts highly relevant to stakeholders, policy makers and public communication: such as biodiversity, environmental threat and ecosystem services. A clear conclusion of the review is that advances in both sensor technology and processing algorithms continue to drive forward remote sensing capability for coral reef mapping, particularly with respect to spatial resolution of maps, and synthesis across multiple data products. Both trends can be expected to continue.
Marine and Freshwater Research, 2012
Understanding the nature and causes of recent climate variability on the Great Barrier Reef (GBR)... more Understanding the nature and causes of recent climate variability on the Great Barrier Reef (GBR), Australia, is fundamental to assessing the impacts of future climate change on this complex ecosystem. New analytical tools, improved data quality and resolution, longer time-series and new variables provide an opportunity to re-assess existing paradigms. Here, we examined sea surface temperature (SST), sea level pressure, surface winds, sea surface height and ocean currents for the period from 1948 to 2009. We focussed on the relationship between GBR surface climate and the wider tropical Pacific, and the influence of El Niño-Southern Oscillation (ENSO) events. Also, for the first time, we investigated the impact of the El Niño/La Niña Modoki phenomenon. Although neither type of ENSO event is a primary driver of inter-annual climate variability on the GBR, their influence is conspicuous. Classical ENSO events have a strong signature in the atmospheric circulation in the northern GBR but no significant relationship with SSTs and the opposite applies for the southern GBR. Conversely, El Niño/La Niña Modoki is significantly related to summer SSTs on the northern GBR, but not for the southern GBR. This study enhances our understanding of tropical Pacific and GBR climate drivers and will improve future predictions of change in climate variables that are likely to impact on the complex GBR ecosystem.
Fluorometers are widely used in ecosystem observing to monitor fluorescence signals from organic ... more Fluorometers are widely used in ecosystem observing to monitor fluorescence signals from organic compounds, as well as to infer geophysical parameters such as chlorophyll or CDOM concentration, but measurements are susceptible to variation caused by biofouling, instrument design, sensor drift, operating environment, and calibration rigor. To collect high quality data, such sensors need frequent checking and regular calibration. In this study, a wide variety of both liquid and solid fluorescent materials were trialed to assess their suitability as reference standards for performance assessment of in situ fluorometers. Criteria used to evaluate the standards included the spectral excitation/emission responses of the materials relative to fluorescence sensors and to targeted ocean properties, the linearity of the fluorometer's optical response with increasing concentration, stability and consistency, availability and ease of use, as well as cost. Findings are summarized as a series of recommended reference standards for sensors deployed on stationary and mobile platforms, to suit a variety of in situ coastal to ocean sensor configurations. Repeated determinations of chlorophyll scale factor using the recommended liquid standard, Fluorescein, achieved an accuracy of 2.5%. Repeated measurements with the recommended solid standard, Plexiglas Satinice ® plum 4H01 DC (polymethylmethacrylate), over an 18 day period varied from the mean value by 1.0% for chlorophyll sensors and 3.3% for CDOM sensors.
Journal of Geophysical Research: Oceans, 2015
Spatial and temporal dynamics of ecological processes have long been considered important in mari... more Spatial and temporal dynamics of ecological processes have long been considered important in marine systems, but seldom have conservation objectives been set for them. Climate change makes the consideration of the dynamics of ecological processes in the design of marine protected areas critical. We analyzed sea-surface temperature (SST) trends and variability in Great Barrier Reef Marine Park (GBRMP) for 25 years and formulated and tested whether three sets of notional conservation objectives were met to illustrate the potential for planning to address climate change. Given mixed and limited evidence that notake areas increase resilience to disturbances such as anomalously high temperatures (i.e., temperatures ≥1 • C above weekly mean temperature), our conservation objectives focused on areas less likely to be affected by such events at extents ranging from the entire Great Barrier Reef to the system of no-take zones and individual no-take zones. The objective sets were (1) at least 50% of temperature refugia (i.e., pixels that had high-temperature anomalies <5% or <7% of the time) within no-take zones, (2) maximum occurrence of high-temperature anomalies is <10%,< 20%, or <30% of total no-take area 90% of the time, and (3) coverage of any single no-take zone by high-temperature anomalies occurs <5% or <10% of the time. We used satellite imagery from 1985-2009 to measure SST to determine high-temperature anomalies. SSTs in the Great Barrier Reef increased significantly in some regions, and some of the conservation objectives were met by the park's current zoning plan. Dialogue between conservation scientists and managers is needed to develop appropriate conservation objectives under climate change and strategies to meet them.
* Deutsches Zentrum für Marine Biodiversität, Martin-Luther-King-Platz 3, 20146 Hamburg, Germany.... more * Deutsches Zentrum für Marine Biodiversität, Martin-Luther-King-Platz 3, 20146 Hamburg, Germany. HCJOHN@zoologie.uni-hamburg.de ** Institut für Ostseeforschung, Seestraße 15, 18119 Warnemünde, Germany. VOLKER.MOHRHOLZ@io-warnemuende.de + ...
ABSTRACT The re-evaluation of historical hydrographic sections of cruises over the Agulhas Bank f... more ABSTRACT The re-evaluation of historical hydrographic sections of cruises over the Agulhas Bank for the period 1974 to 1979, and additional current meter data, elucidated a density flow of nutrient-rich 10oC water along the 100 m isobath on the Agulhas Bank. Two lines of XBT observations were carried out along the 100 m isobath specifically to address aspects of the movement of bottom water across the Agulhas Bank. The first was taken during August-September 1997 from the vessel Seward Johnson during a KAPEX cruise (Boebel et al., 1997); the second on the ACSEX 1 cruise of March 1999 from the Pelagia (Ridderinkhof et al., 2000). Both XBT lines indicated a density flow, although semi-continuous. Recent results of Mitchell-Innes et al. (1999) have located this dense 10oC water mass as far as 35oS, off Walker Bay on the western Agulhas Bank clearly indicating an Indian-Atlantic Interocean secondary gateway. We hypothesize that Delagoa Pulses (Meyer et al., 2002) act as a mechanism for the injection of upwelled Agulhas Undercurrent water masses and Red Sea Water onto the eastern Agulhas Bank, supplying the semi-continuous density flow along the 100 m isobath. This density current originates from the Indian Ocean sector of the Agulhas Bank at the Port Alfred Upwelling Cell, feeds the cold bottom ridge, rounds the Alphard Banks, and enters the Atlantic Ocean sector of the Agulhas Bank. The final destination of this hypothesized current also known as the Lutjeharms (Indian-Atlantic Interocean secondary leakage) Return Current still needs to be investigated.
We examined the spatial and temporal variability of chlorophyll-a (chl-a) concentrations in the c... more We examined the spatial and temporal variability of chlorophyll-a (chl-a) concentrations in the coastal Gulf of Alaska (GOA) using a 14-year time series of cloud-free reconstructions of SeaWiFS and MODIS-Aqua chl-a data. Coast-wide and regional relationships between chl-a and likely environmental drivers, including anomalies in sea surface temperature (SST), photosynthetically-available radiation (PAR), sea surface height (SSHa), freshwater discharge, and coastal upwelling were explored. Coast-wide chl-a showed strong seasonal variability with pronounced spring and fall blooms, but both the magnitude and the seasonal patterns showed considerable interannual variability. Coast-wide annual chl-a anomalies were positive in years with elevated PAR, low SST, and a reduction in downwelling-favorable windsconditions that are indicative of reduced cyclonic circulation associated with a weak Aleutian Low. The apparent negative effect of strong cyclonic circulation in the GOA on chl-a levels was further supported by negative relationships between elevated sea levels and coast-wide chl-a in both the spring and fall. If chl-a concentrations in the coastal GOA reflect productivity, these results are contrary to the prevailing view that strong cyclonic circulation enhances productivity in the GOA. The variability in the chl-a in each of four distinct and spatially contiguous regions that differed in the timing and magnitude of the spring and fall blooms was associated with different combinations of environmental variables.
Large tropical and subtropical marine animals must meet their energetic requirements in a largely... more Large tropical and subtropical marine animals must meet their energetic requirements in a largely oligotrophic environment. Many planktivorous elasmobranchs, whose thermal ecolo-gies prevent foraging in nutrient-rich polar waters, aggregate seasonally at predictable locations throughout tropical oceans where they are observed feeding. Here we investigate the foraging and oceanographic environment around Lady Elliot Island, a known aggregation site for reef manta rays Manta alfredi in the southern Great Barrier Reef. The foraging behaviour of reef manta rays was analysed in relation to zooplankton populations and local oceanography, and compared to long-term sighting records of reef manta rays from the dive operator on the island. Reef manta rays fed at Lady Elliot Island when zooplankton biomass and abundance were significantly higher than other times. The critical prey density threshold that triggered feeding was 11.2 mg m-3 while zooplankton size had no significant effect on feeding. The community composition and size structure of the zooplankton was similar when reef manta rays were feeding or not, with only the density of zooplankton changing. Higher zooplankton biomass was observed prior to low tide, and long-term (~5 years) sighting data confirmed that more reef manta rays are also observed feeding during this tidal phase than other times. This is the first study to examine prey availability at an aggregation site for reef manta rays and it indicates that they feed in locations and at times of higher zoo-plankton biomass.
Coral reefs are in decline worldwide and monitoring activities are important for assessing the im... more Coral reefs are in decline worldwide and monitoring activities are important for assessing the impact of disturbance on reefs and tracking subsequent recovery or decline. Monitoring by field surveys provides accurate data but at highly localised scales and so is not cost-effective for reef scale monitoring at frequent time points. Remote sensing from satellites is an alternative and complementary approach. While remote sensing cannot provide the level of detail and accuracy at a single point than a field survey, the statistical power for inferring large scale patterns benefits in having complete areal coverage. This review considers the state of the art of coral reef remote sensing for the diverse range of objectives relevant for management, ranging from the composition of the reef: physical extent, benthic cover, bathymetry, rugosity; to environmental parameters: sea surface temperature, exposure, light, carbonate chemistry. In addition to updating previous reviews, here we also consider the capability to go beyond basic maps of habitats or environmental variables, to discuss concepts highly relevant to stakeholders, policy makers and public communication: such as biodiversity, environmental threat and ecosystem services. A clear conclusion of the review is that advances in both sensor technology and processing algorithms continue to drive forward remote sensing capability for coral reef mapping, particularly with respect to spatial resolution of maps, and synthesis across multiple data products. Both trends can be expected to continue.
Marine and Freshwater Research, 2012
Understanding the nature and causes of recent climate variability on the Great Barrier Reef (GBR)... more Understanding the nature and causes of recent climate variability on the Great Barrier Reef (GBR), Australia, is fundamental to assessing the impacts of future climate change on this complex ecosystem. New analytical tools, improved data quality and resolution, longer time-series and new variables provide an opportunity to re-assess existing paradigms. Here, we examined sea surface temperature (SST), sea level pressure, surface winds, sea surface height and ocean currents for the period from 1948 to 2009. We focussed on the relationship between GBR surface climate and the wider tropical Pacific, and the influence of El Niño-Southern Oscillation (ENSO) events. Also, for the first time, we investigated the impact of the El Niño/La Niña Modoki phenomenon. Although neither type of ENSO event is a primary driver of inter-annual climate variability on the GBR, their influence is conspicuous. Classical ENSO events have a strong signature in the atmospheric circulation in the northern GBR but no significant relationship with SSTs and the opposite applies for the southern GBR. Conversely, El Niño/La Niña Modoki is significantly related to summer SSTs on the northern GBR, but not for the southern GBR. This study enhances our understanding of tropical Pacific and GBR climate drivers and will improve future predictions of change in climate variables that are likely to impact on the complex GBR ecosystem.
Fluorometers are widely used in ecosystem observing to monitor fluorescence signals from organic ... more Fluorometers are widely used in ecosystem observing to monitor fluorescence signals from organic compounds, as well as to infer geophysical parameters such as chlorophyll or CDOM concentration, but measurements are susceptible to variation caused by biofouling, instrument design, sensor drift, operating environment, and calibration rigor. To collect high quality data, such sensors need frequent checking and regular calibration. In this study, a wide variety of both liquid and solid fluorescent materials were trialed to assess their suitability as reference standards for performance assessment of in situ fluorometers. Criteria used to evaluate the standards included the spectral excitation/emission responses of the materials relative to fluorescence sensors and to targeted ocean properties, the linearity of the fluorometer's optical response with increasing concentration, stability and consistency, availability and ease of use, as well as cost. Findings are summarized as a series of recommended reference standards for sensors deployed on stationary and mobile platforms, to suit a variety of in situ coastal to ocean sensor configurations. Repeated determinations of chlorophyll scale factor using the recommended liquid standard, Fluorescein, achieved an accuracy of 2.5%. Repeated measurements with the recommended solid standard, Plexiglas Satinice ® plum 4H01 DC (polymethylmethacrylate), over an 18 day period varied from the mean value by 1.0% for chlorophyll sensors and 3.3% for CDOM sensors.
Journal of Geophysical Research: Oceans, 2015
Spatial and temporal dynamics of ecological processes have long been considered important in mari... more Spatial and temporal dynamics of ecological processes have long been considered important in marine systems, but seldom have conservation objectives been set for them. Climate change makes the consideration of the dynamics of ecological processes in the design of marine protected areas critical. We analyzed sea-surface temperature (SST) trends and variability in Great Barrier Reef Marine Park (GBRMP) for 25 years and formulated and tested whether three sets of notional conservation objectives were met to illustrate the potential for planning to address climate change. Given mixed and limited evidence that notake areas increase resilience to disturbances such as anomalously high temperatures (i.e., temperatures ≥1 • C above weekly mean temperature), our conservation objectives focused on areas less likely to be affected by such events at extents ranging from the entire Great Barrier Reef to the system of no-take zones and individual no-take zones. The objective sets were (1) at least 50% of temperature refugia (i.e., pixels that had high-temperature anomalies <5% or <7% of the time) within no-take zones, (2) maximum occurrence of high-temperature anomalies is <10%,< 20%, or <30% of total no-take area 90% of the time, and (3) coverage of any single no-take zone by high-temperature anomalies occurs <5% or <10% of the time. We used satellite imagery from 1985-2009 to measure SST to determine high-temperature anomalies. SSTs in the Great Barrier Reef increased significantly in some regions, and some of the conservation objectives were met by the park's current zoning plan. Dialogue between conservation scientists and managers is needed to develop appropriate conservation objectives under climate change and strategies to meet them.
* Deutsches Zentrum für Marine Biodiversität, Martin-Luther-King-Platz 3, 20146 Hamburg, Germany.... more * Deutsches Zentrum für Marine Biodiversität, Martin-Luther-King-Platz 3, 20146 Hamburg, Germany. HCJOHN@zoologie.uni-hamburg.de ** Institut für Ostseeforschung, Seestraße 15, 18119 Warnemünde, Germany. VOLKER.MOHRHOLZ@io-warnemuende.de + ...
ABSTRACT The re-evaluation of historical hydrographic sections of cruises over the Agulhas Bank f... more ABSTRACT The re-evaluation of historical hydrographic sections of cruises over the Agulhas Bank for the period 1974 to 1979, and additional current meter data, elucidated a density flow of nutrient-rich 10oC water along the 100 m isobath on the Agulhas Bank. Two lines of XBT observations were carried out along the 100 m isobath specifically to address aspects of the movement of bottom water across the Agulhas Bank. The first was taken during August-September 1997 from the vessel Seward Johnson during a KAPEX cruise (Boebel et al., 1997); the second on the ACSEX 1 cruise of March 1999 from the Pelagia (Ridderinkhof et al., 2000). Both XBT lines indicated a density flow, although semi-continuous. Recent results of Mitchell-Innes et al. (1999) have located this dense 10oC water mass as far as 35oS, off Walker Bay on the western Agulhas Bank clearly indicating an Indian-Atlantic Interocean secondary gateway. We hypothesize that Delagoa Pulses (Meyer et al., 2002) act as a mechanism for the injection of upwelled Agulhas Undercurrent water masses and Red Sea Water onto the eastern Agulhas Bank, supplying the semi-continuous density flow along the 100 m isobath. This density current originates from the Indian Ocean sector of the Agulhas Bank at the Port Alfred Upwelling Cell, feeds the cold bottom ridge, rounds the Alphard Banks, and enters the Atlantic Ocean sector of the Agulhas Bank. The final destination of this hypothesized current also known as the Lutjeharms (Indian-Atlantic Interocean secondary leakage) Return Current still needs to be investigated.
We examined the spatial and temporal variability of chlorophyll-a (chl-a) concentrations in the c... more We examined the spatial and temporal variability of chlorophyll-a (chl-a) concentrations in the coastal Gulf of Alaska (GOA) using a 14-year time series of cloud-free reconstructions of SeaWiFS and MODIS-Aqua chl-a data. Coast-wide and regional relationships between chl-a and likely environmental drivers, including anomalies in sea surface temperature (SST), photosynthetically-available radiation (PAR), sea surface height (SSHa), freshwater discharge, and coastal upwelling were explored. Coast-wide chl-a showed strong seasonal variability with pronounced spring and fall blooms, but both the magnitude and the seasonal patterns showed considerable interannual variability. Coast-wide annual chl-a anomalies were positive in years with elevated PAR, low SST, and a reduction in downwelling-favorable windsconditions that are indicative of reduced cyclonic circulation associated with a weak Aleutian Low. The apparent negative effect of strong cyclonic circulation in the GOA on chl-a levels was further supported by negative relationships between elevated sea levels and coast-wide chl-a in both the spring and fall. If chl-a concentrations in the coastal GOA reflect productivity, these results are contrary to the prevailing view that strong cyclonic circulation enhances productivity in the GOA. The variability in the chl-a in each of four distinct and spatially contiguous regions that differed in the timing and magnitude of the spring and fall blooms was associated with different combinations of environmental variables.