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  1. Abstract

    Ammonium, a key intermediate nutrient, is typically low to undetectable on the Oregon coast, particularly as active upwelling delivers high onshore flow of ammonium‐poor waters. However, during bloom and post‐bloom conditions large ammonium concentrations and uptake rates have been described. High‐frequencyon boardnitrate + nitrite and ammonium analysis synchronized with continuous data from a towed profiling vehicle (equipped with in situ temperature, salinity, dissolved oxygen, and beam attenuation sensors), allowed us to describe coupled high‐resolution physico‐chemical dynamics of inorganic nitrogen in seven cross‐shelf transects, over several days, during an active phytoplankton bloom following cessation of upwelling favorable winds. We present first‐of‐their‐kind high‐resolution cross‐sections showing a build‐up, both within a thin plume of onshore‐originated water, and in mid‐to‐bottom on‐shore water columns, from undetectable values to up to 8 µM of ammonium. The plume extended across the shelf at mid‐depth and was identified in all transects. We also detected a decrease of nitrate in distinct water masses close to the mid‐shelf seabed, associated with low dissolved oxygen, and identified and quantified the amount of nitrogen lost. We found that nitrogen loss was minimal on the first days of relaxation conditions, and increased up to 12 μM off Newport. Combining nitrogen fluxes from benthic incubation chambers and N loss calculated with the NO tracer, we estimated that denitrification in sediments could not account for all N loss, requiring 22%–86% to occur elsewhere. Close association of N loss with particle‐rich, low O2waters suggests the possibility that particle‐aggregate micro‐environments could provide additional sites for water column denitrification.

     
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  2. Abstract

    A realistic numerical model is used to study the circulation and mixing of the Salish Sea, a large, complex estuarine system on the United States and Canadian west coast. The Salish Sea is biologically productive and supports many important fisheries but is threatened by recurrent hypoxia and ocean acidification, so a clear understanding of its circulation patterns and residence times is of value. The estuarine exchange flow is quantified at 39 sections over 3 years (2017–2019) using the Total Exchange Flow method. Vertical mixing in the 37 segments between sections is quantified as opposing vertical transports: the efflux and reflux. Efflux refers to the rate at which deep, landward‐flowing water is mixed up to become part of the shallow, seaward‐flowing layer. Similarly, reflux refers to the rate at which upper layer water is mixed down to form part of the landward inflow. These horizontal and vertical transports are used to create a box model to explore residence times in a number of different sub‐volumes, seasons, and years. Residence times from the box model are generally found to be longer than those based on simpler calculations of flushing time. The longer residence times are partly due to reflux, and partly due to incomplete tracer homogenization in sub‐volumes. The methods presented here are broadly applicable to other estuaries.

     
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  3. Abstract

    The southern Benguela upwelling system (SBUS) supports high rates of primary productivity that sustain important commercial fisheries. The exceptional fertility of this system is reportedly fueled not only by upwelled nutrients but also by nutrients regenerated on the broad and shallow continental shelf. We measured nutrient concentrations and the nitrogen (N) and oxygen (O) isotope ratios (δ15N and δ18O) of nitrate along four zonal lines in the SBUS in late summer and early winter to evaluate the extent to which regenerated nutrients augment the upwelled nutrient reservoir originating offshore. During summer upwelling, a decrease in on‐shelf nitrate δ18O revealed that 0–48% of the subsurface nutrients derived from in situ remineralization. The nitrate regenerated on‐shelf in the more quiescent winter (0–63% of total nitrate) extended further offshore along the mid‐shelf. A shoreward increase in subsurface nitrate δ15N and a greater N deficit in on‐shelf bottom waters further indicated N loss to benthic (and at times, watercolumn) denitrification coincident with the on‐shelf remineralization. Our data show that remineralized nutrients get trapped on the SBUS shelf in summer through early winter, enhancing the nutrient pool that can be upwelled to support surface production. We hypothesize that this process is aided by a number of equatorward‐flowing hydrographic fronts that impede the lateral exchange of surface waters. The extent to which nutrients remain trapped on the shelf has implications for the occurrence of hypoxic events in the SBUS.

     
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