Abstract
The spatial and temporal variability of phytoplankton blooms was investigated in two tropical coastal regions of northern Australia using the MEdium Resolution Imaging Spectrometer (MERIS) full mission (2002–2012) reduced resolution dataset. Satellite-derived proxies for phytoplankton (Chlorophyll-a (Chl), Fluorescence Line Height (FLH), Maximum Chlorophyll Index (MCI)) and suspended sediment (Total Suspended Matter (TSM)) were jointly analyzed for two clusters of the Great Barrier Reef Wet tropics (GBRW; 15°–19.5°S; Queensland) and the Van Diemen Gulf (VDG; 9°–13°S; Northern Territory). The analysis of time-series and Hovmöller diagrams of the four MERIS products provided a unique perspective on the processes linking phytoplankton blooms and river runoff, or resuspension, across spatio-temporal scales. Both regions are characterized by a complex oceanography and seasonal inflows of sediment, freshwater and nutrients during the tropical wet season months (November to April). The GBRW is characterized by a great variability in water clarity (Secchi depth 0–25 m). A long history of agricultural land use has led to a large increase in the seasonal discharge of sediments and nutrients, triggering seasonal phytoplankton blooms (>0.4 mg∙m−3) between January and April. In contrast, the VDG is a poorly flushed, turbid (Secchi depth <5 m) environment with strong tidal-energy (4–8 m) and very limited land use. Phytoplankton blooms here were found to have higher Chl concentrations (>1.0 mg∙m−3) than in the GBRW, occurring up to twice a year between January and April. Over the 10-year MERIS mission, a weak decline in Chl and TSM was observed for the VDG (Sen slope: −2.85%/decade, τ = −0.32 and −3.57%/decade, τ = −0.24; p 0.05), while no significant trend in those two satellite products was observed in the GBRW. Cyanobacteria surface algal blooms occur in both regions between August and October. The MCI and FLH products were found to adequately complement Chl, while TSM provided relevant insight for the assessment of sediment resuspension and river runoff.
Highlights
Spatial and temporal mapping of phytoplankton blooms has been undertaken in many parts of the world’s oceans, with a major emphasis on temperate coastal and oceanic waters where an annual spring bloom occurs [1,2]
Phytoplankton dynamics in tropical marine systems remains largely understudied. To address this knowledge gap, this study investigated the spatio-temporal distribution of phytoplankton blooms by means of satellite proxies in the context of the tropical waters of the Northern Great Barrier Reef (GBR, North Queensland) and the Van Diemen Gulf
This study aims at demonstrating that the combined analysis of four ocean color products over the full mission of a single sensor can be used to unravel the dynamics of phytoplankton in a coastal region
Summary
Spatial and temporal mapping of phytoplankton blooms has been undertaken in many parts of the world’s oceans, with a major emphasis on temperate coastal and oceanic waters where an annual spring bloom occurs [1,2]. Phytoplankton dynamics in tropical marine systems remains largely understudied. To address this knowledge gap, this study investigated the spatio-temporal distribution of phytoplankton blooms by means of satellite proxies in the context of the tropical waters of the Northern Great Barrier Reef (GBR, North Queensland) and the Van Diemen Gulf The coastal regions studied are Cairns (15.1°–19.5°S), located in the Great. Both study regions are intricate, bio-diverse coastal ecosystems with a complex oceanography [3,4]. The GBR is at the forefront of this situation [7], with watershed-based pollution from adjacent agricultural land use [8,9] and thermal stress affecting its coral ecosystem [10]
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