Unveiling a phytoplankton hotspot at a narrow boundary between coastal and offshore waters

Abstract

In terrestrial ecosystems, transitional areas between different plant communities (ecotones) are formed by steep environmental gradients and are commonly characterized by high species diversity and primary productivity, which in turn influences the foodweb structure of these regions. Whether comparable zones of elevated diversity and productivity characterize ecotones in the oceans remains poorly understood. Here we describe a previously hidden hotspot of phytoplankton diversity and productivity in a narrow but seasonally persistent transition zone at the intersection of iron-poor, nitrate-rich offshore waters and iron-rich, nitrate-poor coastal waters of the Northeast Pacific Ocean. Novel continuous measurements of phytoplankton cell abundance and composition identified a complex succession of blooms of five distinct size classes of phytoplankton populations within a 100-km-wide transition zone. The blooms appear to be fueled by natural iron enrichment of offshore communities as they are transported toward the coast. The observed succession of phytoplankton populations is likely driven by spatial gradients in iron availability or time since iron enrichment. Regardless of the underlying mechanism, the resulting communities have a strong impact on the regional biogeochemistry as evidenced by the low partial pressure of CO 2 and the nearly complete depletion of nutrients. Enhanced phytoplankton productivity and diversity associated with steep environmental gradients are expected wherever water masses with complementary nutrient compositions mix to create a region more favorable for phytoplankton growth. The ability to detect and track these important but poorly characterized marine ecotones is critical for understanding their impact on productivity and ecosystem structure in the oceans.ecotone | transition zone | iron | high-nitrate, low-chlorophyll | flow cytometry

Diatom Transcriptional and Physiological Responses to Changes in Iron Bioavailability across Ocean Provinces

“…In this region, Fe is supplied to surface waters mainly through atmospheric deposition of dust from arid continental regions and volcanic emissions, with Fe inputs from continental margin sediments fueling winter phytoplankton blooms when atmospheric deposition is low (Lam et al, 2006;Lam and Bishop, 2008). A gradient in surface nutrient concentrations is observed from this oceanic region eastwards toward the continent; bioavailable Fe increases and supports higher phytoplankton biomass while NO − 3 concentrations in the upper mixed layer decrease to limiting levels on the continental shelf (Taylor and Haigh, 1996;Harris et al, 2009;Ribalet et al, 2010).…”

Comparative metatranscriptomics identifies molecular bases for the physiological responses of phytoplankton to varying iron availability

“…Nitrate concentrations continued to decline in the plusFe sample after 98 h, indicating that at the time of sampling, the iron-stimulated phytoplankton community was in the early to midstages of bloom development. On the basis of continuous flow cytometry measurements in parallel iron-enriched incubation experiments, these large cells peaked in cell abundance on day 5 (∼120 h), at which time NO 3 − concentrations were fully depleted (25). A modest, yet significant increase in phytoplankton biomass in the Ctl was observed compared with that in the ambient waters, possibly resulting from changes in light conditions, minor iron contamination, and/or reductions in grazing pressure associated with the incubations.…”

“…‡ Size classes of phytoplankton populations are described in Ribalet et al (25). § Pennate diatoms such as Pseudo-nitzschia spp.…”

Seasonal and interannual variability of the marine bacterioplankton community throughout the water column over ten years

“…The biodiversity maximum in the mid-water column at 150 m (Figure 3a) reflects patterns seen elsewhere in the ocean for other organisms including copepods, ostracods (Angel, 1993;Lindsay and Hunt, 2005) and fish (Badcock and Merrett, 1976). Biodiversity is often highest at ecotones, the interface between different environments, (Angel, 1993;Barton et al, 2010;Ribalet et al, 2010) and the 150 m depth is such an interface between the euphotic and disphotic zones.…”