Environmental and biological factors controlling the spring phytoplankton bloom at the Patagonian shelf-break front – Degraded fucoxanthin pigments and the importance of microzooplankton grazing

Abstract

The aim of this study was to investigate the biotic and abiotic factors controlling the spring phytoplankton blooms at the Patagonian shelf-break front (PSBF). Using a CHEMTAX analysis of HPLC pigment data and other methods, the biomass and spatial variability of plankton communities were studied in four sections (39-48°S) across the PSBF during October 2005. Environmental factors and the biomass and composition of plankton communities exhibited a marked spatial heterogeneity. The latitudinal and cross-shelf progression in the timing of the spring bloom initiation and the nutritive properties of the water masses (Subantarctic Shelf Waters and Malvinas Current Waters) seemed to be the key factors. Three plankton regions were distinguished: a) Outer shelf (OS) b) Shelf-break front (SBF) and c) Malvinas Current (MC). At the highly stratified OS region, the postbloom community showed low-biomasshigh-phytoplankton diversity formed mainly by small cells (haptophytes 30-62%, diatoms 17-49%, chlorophytes 0-34%, and prasinophytes 0-21% of total Chl a). High amounts of degraded fucoxanthin were found associated with the heterotrophic dinoflagellate, Protoperidinium capurroi. Grazing by this microheterotroph on the diatom population seemed to be the most important factor for the spring bloom decay at the OS. A remarkable quasi monospecific bloom (~90%) of a nanodiatom (Thalassiosira bioculata var.raripora) associated with high Chl a (up to 20 mg m-3) occurred along (~1000 km) the SBF and in the most northern extension of the MC. In the southern region, the bloom was developed under absent or incipient density stratification, increasing solar irradiance, high nitrate and phosphate availability, and low numbers of phytoplankton grazers. The average mixedlayer PAR irradiance (< 2.0 mol quanta PAR m-2 d-1) and Si:N ratios (< 0.2) were low, suggesting a diatom population limited by light and under progressive silicate limitation. The more stratified northern region of the SBF showed a later stage of the bloom development, but the large population of diatoms under Si limitation was not in senescence and losses from microzooplankton grazing were minor. The observed high proportion of Chl a below a shallow upper mixed layer (up to 85 %) could directly reach the bed, favoring the development of epibenthic communities and the formation of seed diatom banks and organic iron-rich sediments. The upwelling along the SBF provides a large source of macronutrients and probably the dissolved iron needed to sustain the intense diatom bloom, but also diatom resting stages that could act as seeds for the next spring bloom. The macronutrient-rich MC region showed low chlorophyll (Chl a< 0.8 mg m-3) and a highly diverse phytoplankton community, mainly composed of small cells (diatoms 20-70%, haptophytes 20-40%, chlorophytes 2-25%, prasinophytes 2-18%, and cryptophytes 3-12% of total Chl a).

Wind modulation of upwelling at the shelf‐break front off Patagonia: Observational evidence

“…Nutrient distributions for the synoptic transects presented in this study are reported by Carreto et al . [, Figure 5], who show higher nutrient concentrations at the SBF for the southerly wind case (i.e., Valdés, see Figures c and d) compared with the northerly wind case (i.e., Rincón, see Figures a and b), in agreement with the results presented here. Due to the lack of concomitant observations of nutrients, vertical diffusivities and/or vertical velocities, a meaningful assessment of the upwelled nutrient fluxes associated with the along‐front wind induced upwelling [e.g., He et al ., ] is not feasible.…”

“…However, the Chl‐a fluorescence difference between subsequent changes in along‐front wind direction can be significantly larger in the spring (±5 mg m −3 ), compared to the summer estimate from satellite observations. Nutrient distributions for the synoptic transects presented in this study [ Carreto et al ., ] are in accord with the theory. High‐resolution (i.e., hourly) wind and water column hydrographic time series collected in spring at the mooring site provide further evidence and show that southerly winds destratify the water column leading to cooling events, whereas northerly winds restratify the upper ocean and lead to warming events.…”

Shionodiscus gaarderaesp. nov. (Thalassiosirales, Thalassiosiraceae), a bloom-producing diatom from the southwestern Atlantic Ocean, and emendation ofShionodiscus bioculatusvar.bioculatus

“…raripora). Quantitative analysis of this bloom revealed that this taxon reached concentrations up to 4.5 × 10 6 cells L −1 , at water temperatures from 5°C to 9°C, representing ∼ 90% of total identified phytoplankton cell concentration, and associated with high concentrations of Chl a (up to 20 mg m −3 ) (Sabatini et al 2012, Carreto et al 2016. A similar bloom was observed in the same area the following spring (Alverson et al 2006), suggesting that spring blooms of S. gaarderae could be recurrent in slope waters of the Argentine Sea.…”

“…raripora) in the Southern Hemisphere was from the southwestern Atlantic Ocean, in slope waters of the Argentine Sea (Ferrario 2008). During 2005, an extensive spring bloom dominated by a small diatom was observed along shelf waters from 39°S to 48°S (Ferrario 2008, Ferrario et al 2012, Sabatini et al 2012, Carreto et al 2016. Analysis by electron microscopy indicated that the dominant species was S. gaarderae (Ferrario et al 2013, as T. bioculata var.…”

Carbon flow through the pelagic food web in southern Chilean Patagonia: relevance of Euphausia vallentini as a key species

“…The high HNF bacterivory rate, and the important role exerted by heterotrophic dinoflagellates, both as prey for euphausiids (Fig. 10) and as diatom grazers (Carreto et al 2016), highlights the importance of the microbial food web. In general, because of the high complexity of the food web, more information from integrative studies on end-to-end food web characteristics and organic carbon export in Patagonian fjords and channels is required.…”