Abstract
The deployment of animal-borne electronic tags is revolutionizing our understanding of how pelagic species respond to their environment by providing in situ oceanographic information such as temperature, salinity, and light measurements. These tags, deployed on pelagic animals, provide data that can be used to study the ecological context of their foraging behaviour and surrounding environment. Satellite-derived measures of ocean colour reveal temporal and spatial variability of surface chlorophyll-a (a useful proxy for phytoplankton distribution). However, this information can be patchy in space and time resulting in poor correspondence with marine animal behaviour. Alternatively, light data collected by animal-borne tag sensors can be used to estimate chlorophyll-a distribution. Here, we use light level and depth data to generate a phytoplankton index that matches daily seal movements. Time-depth-light recorders (TDLRs) were deployed on 89 southern elephant seals (Mirounga leonina) over a period of 6 years (1999–2005). TDLR data were used to calculate integrated light attenuation of the top 250 m of the water column (LA250), which provided an index of phytoplankton density at the daily scale that was concurrent with the movement and behaviour of seals throughout their entire foraging trip. These index values were consistent with typical seasonal chl-a patterns as measured from 8-daySea-viewing Wide Field-of-view Sensor (SeaWiFs) images. The availability of data recorded by the TDLRs was far greater than concurrent remotely sensed chl-a at higher latitudes and during winter months. Improving the spatial and temporal availability of phytoplankton information concurrent with animal behaviour has ecological implications for understanding the movement of deep diving predators in relation to lower trophic levels in the Southern Ocean. Light attenuation profiles recorded by animal-borne electronic tags can be used more broadly and routinely to estimate lower trophic distribution at sea in relation to deep diving predator foraging behaviour.
Highlights
Chlorophyll-a is an important biological parameter in the Southern Ocean and is considered a useful indicator of spatial and temporal variability of primary productivity [1,2,3]
This study examined the feasibility of using light collected from Time-depth-light recorders (TDLRs) to calculate an index of phytoplankton distribution that is concurrent with marine animal behaviour in the Southern Ocean
Of the 3940 light attenuation at 250 m (LA250) gridded cells recorded over 5 years, only 25.1% cells coincided with chl-a measurements demonstrating the deficiency of remotely sensed data sources concurrent with animal behaviour
Summary
Chlorophyll-a is an important biological parameter in the Southern Ocean and is considered a useful indicator of spatial and temporal variability of primary productivity [1,2,3]. Satellite measurements of ocean colour have revealed the complex temporal and spatial variability of weighted average nearsurface chlorophyll-a concentration [4], but the quantity and quality of information obtained in this way is affected by cloud cover. While fluorometers and water samples from ship-based surveys are the only in-vivo and in-vitro measurements to determine chlorophyll-a concentration, it is both costly and logistically difficult if collecting simultaneously with animal behaviour. Additional ocean data recorded by animal-borne electronic tags have been used to supplement other data from buoys and satellites (e.g., [7,8]) and have improved our understanding of the relationship between marine predator distribution and environmental parameters, including chlorophyll-a [9,10]. Understanding lower trophic variability (i.e. phytoplankton) and its influence on marine predators in the Southern Ocean is still hampered by a lack of concurrent data
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