William P. Cochlan

Processes influencing seasonal hypoxia in the northern California Current System

This paper delineates the role of physical and biological processes contributing to hypoxia, dissolved oxygen (DO) < 1.4 mL/L, over the continental shelf of Washington State in the northern portion of the California Current System (CCS). In the historical record (1950-1986) during the summer upwelling season, hypoxia is more prevalent and severe off Washington than further south off northern Oregon. Recent data (2003-2005) show that hypoxia over the Washington shelf occurred at levels previously observed in the historical data. 2006 was an exception, with hypoxia covering ~5000 km(2) of the Washington continental shelf and DO concentrations below 0.5 mL/L at the inner shelf, lower than any known previous observations at that location. In the four years studied, upwelling of low DO water and changes in source water contribute to interannual variability, but cannot account for seasonal decreases below hypoxic concentrations. Deficits of DO along salinity surfaces, indicating biochemical consumption of DO, vary significantly between surveys, accounting for additional decreases of 0.5-2.5 mL/L by late summer. DO consumption is associated with denitrification, an indicator of biochemical sediment processes. Mass balances of DO and nitrate show that biochemical processes in the water column and sediments each contribute ~50% to the total consumption of DO in near-bottom water. At shorter than seasonal time scales on the inner shelf, along-shelf advection of hypoxic patches and cross-shelf advection of seasonal gradients are both shown to be important, changing DO concentrations by 1.5 mL/L or more over five days.

An unprecedented coastwide toxic algal bloom linked to anomalous ocean conditions

Abstract A coastwide bloom of the toxigenic diatom Pseudo‐nitzschia in spring 2015 resulted in the largest recorded outbreak of the neurotoxin, domoic acid, along the North American west coast. Elevated toxins were measured in numerous stranded marine mammals and resulted in geographically extensive and prolonged closures of razor clam, rock crab, and Dungeness crab fisheries. We demonstrate that this outbreak was initiated by anomalously warm ocean conditions. Pseudo‐nitzschia australis thrived north of its typical range in the warm, nutrient‐poor water that spanned the northeast Pacific in early 2015. The seasonal transition to upwelling provided the nutrients necessary for a large‐scale bloom; a series of spring storms delivered the bloom to the coast. Laboratory and field experiments confirming maximum growth rates with elevated temperatures and enhanced toxin production with nutrient enrichment, together with a retrospective analysis of toxic events, demonstrate the potential for similarly devastating ecological and economic disruptions in the future.

Iron enrichment stimulates toxic diatom production in high-nitrate, low-chlorophyll areas

Oceanic high-nitrate, low-chlorophyll environments have been highlighted for potential large-scale iron fertilizations to help mitigate global climate change. Controversy surrounds these initiatives, both in the degree of carbon removal and magnitude of ecosystem impacts. Previous open ocean enrichment experiments have shown that iron additions stimulate growth of the toxigenic diatom genus Pseudonitzschia. Most Pseudonitzschia species in coastal waters produce the neurotoxin domoic acid (DA), with their blooms causing detrimental marine ecosystem impacts, but oceanic Pseudonitzschia species are considered nontoxic. Here we demonstrate that the sparse oceanic Pseudonitzschia community at the high-nitrate, low-chlorophyll Ocean Station PAPA (50° N, 145° W) produces approximately 200 pg DA L−1 in response to iron addition, that DA alters phytoplankton community structure to benefit Pseudonitzschia, and that oceanic cell isolates are toxic. Given the negative effects of DA in coastal food webs, these findings raise serious concern over the net benefit and sustainability of large-scale iron fertilizations.

Ocean urea fertilization for carbon credits poses high ecological risks

The proposed plan for enrichment of the Sulu Sea, Philippines, a region of rich marine biodiversity, with thousands of tonnes of urea in order to stimulate algal blooms and sequester carbon is flawed for multiple reasons. Urea is preferentially used as a nitrogen source by some cyanobacteria and dinoflagellates, many of which are neutrally or positively buoyant. Biological pumps to the deep sea are classically leaky, and the inefficient burial of new biomass makes the estimation of a net loss of carbon from the atmosphere questionable at best. The potential for growth of toxic dinoflagellates is also high, as many grow well on urea and some even increase their toxicity when grown on urea. Many toxic dinoflagellates form cysts which can settle to the sediment and germinate in subsequent years, forming new blooms even without further fertilization. If large-scale blooms do occur, it is likely that they will contribute to hypoxia in the bottom waters upon decomposition. Lastly, urea production requires fossil fuel usage, further limiting the potential for net carbon sequestration. The environmental and economic impacts are potentially great and need to be rigorously assessed.

Impacts on phytoplankton biomass and productivity in the Pacific Northwest during the warm ocean conditions of 2005

[1] Delayed onset of the spring transition and upwellingfavorable winds in the Pacific Northwest during springsummer 2005 resulted in a positive temperature anomaly and a pronounced negative anomaly in surface phytoplankton biomass (chlorophyll a )a nd primary productivity. Compared to time periods before and after the warm water event, total biomass was reduced by ca. 50% along a hydrographic line extending seaward from Grays Harbor, WA (47 N), with a concomitant decrease of ca. 40% in surface and depth-integrated primary productivity. Associated with these declines in biomass and productivity was a change in mean phytoplankton size, with >50% of the nearshore assemblage less than 5m mi n size during the warm event, compared to <30% during more normal conditions. Unlike higher trophic levels, the phytoplankton rapidly recovered with the onset of upwelling, returning to more typical size structure, biomass, and productivity within one week of the onset of upwelling-favorable winds. Citation: Kudela, R. M., W. P. Cochlan, T. D. Peterson, and C. G. Trick (2006), Impacts on phytoplankton biomass and productivity in the Pacific Northwest during the warm ocean conditions of 2005, Geophys. Res. Lett., 33, L22S06, doi:10.1029/2006GL026772.

Seasonal study of uptake and regeneration of nitrogen on the Scotian Shelf

Abstract Nitrate and ammonium uptake and regeneration rates were measured in the euphotic zone of the Scotian Shelf during three cruises (spring, summer and late winter). Nitrate, as a portion of the total nitrogen assimilated (NO 3 − uptake/total (NO 3 − + NH 4 − ) uptake), decreased with increasing ambient NH 4 + concentration and depth. Values integrated through the euphotic zone averaged 30% in the summer, and 27% in the spring, indicating that a large portion of phytoplankton growth was supported by ‘regenerated’ production (NH 4 + ) during those periods. In winter, growth was supported primarily by ‘new’ production since NO 3 − uptake represented 67% of the total nitrogen uptake during that period. In all seasons the phytoplankton showed a consistent preference for NH 4 + utilization relative to NO 3 , despite the abundance of NO 3 at times. In 21 of 23 measurements, NH 4 + remineralization exceeded uptake, suggesting that phytoplankton nitrogen requirements were met or exceeded by in situ NH 4 + regeneration. Remineralization rates covaried with both productivity ( 14 C) and NH 4 + uptake rates within the euphotic zone. These relationships were most apparent during the summer when nitrogen and carbon fluxes and algal biomass (Chl a ) were greatest. The experimental approach used in this study demonstrates a seasonal pattern of NH 4 + and NO 3 − utilization previously unreported for Scotian Shelf waters.

Kinetics of nitrogen (nitrate, ammonium and urea)uptake by the picoflagellate Micromonas pusilla (Prasinophyceae)

Abstract The kinetics of N (nitrate, ammonium and urea) uptake by the eucaryotic picoflagellate Micromonas pusilla (Butcher) Manton et Parke were investigated using N-replete batch cultures and the 15 N tracer technique. Maximum specific uptake rate ( V max ) of NH 4 + was 0.13·h −1 , > 2 times the V max of NO 3 − or urea (≈ 0.05·h −1 ). The half-saturation constants ( K s ) for urea, NH 4 + and NO 3 − were similar with an average value of 0.4 μg-at N·1 −1 which is within the range reported for small oceanic diatoms. The greater initial slope of the Michaelis-Menten plot for NH 4 + uptake suggests that M. pusilla can utilize low concentrations of NH 4 + more effectively than equivalent concentrations of urea or NO 3 − . The results of this first study of N uptake kinetics by an eucaryotic picoplankter demonstrate the preference for NH 4 + over urea and NO 3 − as a nitrogenous nutrient source, which may contribute to the relative success of picoplankton in oligotrophic oceanic regions.

Inhibition of nitrate uptake by ammonium and urea in the eucaryotic picoflagellate Micromonas pusilla (Butcher) Manton et Parke

Experiments were carried out with nitrate-replete cultures of the picoflagellate Micromonas pusilla (Butcher) Manton et Parke to investigate the effects of ammonium and urea additions on nitrate uptake. By employing both 15N methodology and by measuring the ambient nutrient concentrations over time, the uptake rates of the three substrata could be determined. Nitrate uptake was completely inhibited by NH+4 concentrations of 1–10 μg-at N·1−1, whereas a 10 μg-at N·1−1 perturbation of urea resulted in a 28% reduction of NO−3 uptake and both N substrata were taken up simultaneously. This is the first study of the uptake interaction(s) of multiple N substrata for a picoplankter.

Nitrogen Utilization and Toxin Production by Two Diatoms of the Pseudo‐nitzschia pseudodelicatissima Complex: P. cuspidata and P. fryxelliana

The toxigenic diatom Pseudo‐nitzschia cuspidata, isolated from the U.S. Pacific Northwest, was examined in unialgal batch cultures to evaluate domoic acid (DA) toxicity and growth as a function of light, N substrate, and growth phase. Experiments conducted at saturating (120 μmol photons · m−2 · s−1) and subsaturating (40 μmol photons · m−2 · s−1) photosynthetic photon flux density (PPFD), demonstrate that P. cuspidata grows significantly faster at the higher PPFD on all three N substrates tested [nitrate (NO3−), ammonium (NH4+), and urea], but neither cellular toxicity nor exponential growth rates were strongly associated with one N source over the other at high PPFD. However, at the lower PPFD, the exponential growth rates were approximately halved, and the cells were significantly more toxic regardless of N substrate. Urea supported significantly faster growth rates, and cellular toxicity varied as a function of N substrate with NO3−‐supported cells being significantly more toxic than both NH4+‐ and urea‐supported cells at the low PPFD. Kinetic uptake parameters were determined for another member of the P. pseudodelicatissima complex, P. fryxelliana. After growth of these cells on NO3− they exhibited maximum specific uptake rates (Vmax) of 22.7, 29.9, 8.98 × 10−3 · h−1, half‐saturation constants (Ks) of 1.34, 2.14, 0.28 μg‐at N · L−1, and affinity values (α) of 17.0, 14.7, 32.5 × 10−3 · h−1/(μg‐at N · L−1) for NO3−, NH4+ and urea, respectively. These labo‐ratory results demonstrate the capability of P. cuspidata to grow and produce DA on both oxidized and reduced N substrates during both exponential and stationary growth phases, and the uptake kinetic results for the pseudo‐cryptic species, P. fryxelliana suggest that reduced N sources from coastal runoff could be important for maintenance of these small pennate diatoms in U.S. west coast blooms, especially during times of low ambient N concentrations.

Nitrogen Uptake in the Southern Ocean

This chapter summarizes the basic environmental factors regulating nitrogen (N) uptake in the Southern Ocean. These environmental factors include: iron (Fe) and light availability, NH 4 + concentration and its inhibitory effects on NO 3 – uptake, the low ambient seawater temperature, and the role of heterotrophic bacteria as competitors for N in the Southern Ocean. On the basis of the Fe requirements for N assimilation, Fe is more important in the regulation of NO 3 – uptake, and have a lesser effect on the uptake of reduced N substrates (urea and NH 4 + ), although only a few studies have concurrently measured all major substrates as a function of Fe. Given the greater Fe requirements for phytoplankton growth at low light, the regulatory role of Fe is likely accentuated in open ocean areas dominated by deep, wind-mixed layers, and less so in marginal ice zones where low density meltwater stabilizes the water column. The effect(s) of one N source on the utilization of another, namely NH 4 + inhibition of NO 3 – uptake, cannot be ignored, particularly in regions of elevated surface concentrations of NH 4 + . However, the few studies of concentration-dependent experiments demonstrate that even the relatively low concentrations of NH 4 + (0.1–0.2 mM) present throughout the Southern Ocean can potentially reduce NO 3 – uptake by 50% or more. Low ambient temperature undoubtedly factors into the subdued rates of nitrogenous nutrient uptake and primary productivity realized by most pelagic phytoplankton assemblages in the Southern Ocean. Although much progress has been made toward solving the Antarctic Paradox in the last few years, temperature may ultimately prove to be the limiting environmental factor controlling the absolute magnitude of both new and regenerated productivity, whereas the availability of trace metals (notably, Fe), regenerated N sources, and light, appear crucial in dictating their relative importance in the open ocean and seasonal ice zones of the Southern Ocean.