Abstract
Trichodesmium, a filamentous bloom-forming marine cyanobacterium, plays a key role in the biogeochemistry of oligotrophic ocean regions because of the ability to fix nitrogen. Naturally occurring in the Great Barrier Reef (GBR), the contribution of Trichodesmium to the nutrient budget may be of the same order as that entering the system via catchment runoff. However, the cyclicity of Trichodesmium in the GBR is poorly understood and sparsely documented because of the lack of sufficient observations. This study provides the first systematic analysis of Trichodesmium spatial and temporal occurrences in the GBR over the decade-long MERIS ocean color mission (2002–2012). Trichodesmium surface expressions were detected using the Maximum Chlorophyll Index (MCI) applied to MERIS satellite imagery of the GBR lagoonal waters. The MCI performed well (76%), albeit tested on a limited set of images (N = 25) coincident with field measurements. A north (Cape York) to south (Fitzroy) increase in the extent, frequency and timing of the surface expressions characterized the GBR, with surface expressions extending over several hundreds of kilometers. The two southernmost subregions Mackay and Fitzroy accounted for the most (70%) bloom events. The bloom timing of Trichodesmium varied from May in the north to November in the south, with wet season conditions less favorable to Trichodesmium aggregations. MODIS-Aqua Sea Surface Temperature (SST) datasets, wind speed and field measurements of nutrient concentrations were used in combination with MCI positive instances to assess the blooms’ driving factors. Low wind speed (<6 m.s-1) and SST > 24°C were associated with the largest surface aggregations. Generalized additive models (GAM) indicated an increase in bloom occurrences over the 10-year period with seasonal bloom patterns regionally distinct. Interannual variability in SST partially (14%) explained bloom occurrences, and other drivers, such as the subregion and the nutrient budget, likely regulate Trichodesmium surface aggregations in the GBR.
Highlights
Cyanobacterium Trichodesmium sp. is a marine diazotroph [1] phytoplankter found in all tropical and subtropical oceans [2,3,4]
A summary of the physical and chemical variables in the surface waters of each subregion for the period 2002–2013 is presented in S1 Fig. Distinct differences were found in averaged key environmental parameters collected in the five subregions, with the largest spatial variations for temperature, particulate organic carbon (POC), nitrogen (PN), phosphate (PP), dissolved inorganic nitrogen (DIN) and organic phosphate (DOP), total dissolved nitrogen (TDN) and phosphate (TDP)
Most of the nine variables showed slight north to south gradients across the five subregions: temperature decreased from Cape York (CPY) to Fitzroy subregion (FIT) by up to 3.5 ̊C associated with the change in latitude, whereas Chl-a showed little change (s.d.~0.2 μg.L-1) across the subregions
Summary
Cyanobacterium Trichodesmium sp. is a marine diazotroph [1] phytoplankter found in all tropical and subtropical oceans [2,3,4]. Trichodesmium erythraeum, the species occurring in Australian waters, has been reported in the Mediterranean Sea [5] in late summer-early autumn and in UK coastal waters during winter [6]. Trichodesmium is known for forming extensive algal blooms that can cover hundreds of kilometers of the ocean surface. Aggregations have sometimes been so large that the sailors of the HMS Endeavor mistakenly identified a new shoal when the vessel sailed through the Torres Strait (North of Cape York, Australia) in August 1770. These persistent surface expressions occur toward the end of the algal bloom phase [7] and can last several days until the bloom subsides. Composed of senescent cells that are darker than healthy cells due to chlorosis [8], these surface matts increase the water-leaving radiance signal in the red-near-infrared (NIR) spectral region (680–750 nm), called “red edge” [9], allowing their detection from satellite sensors for the past 30 years [10,11,12]
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