Abstract Light plays a critically important role in driving photo‐ and biogeochemistry of aquatic systems, and our ability to measure and model submarine light fields is fairly sophisticated. However, previous approaches have relied almost entirely upon Eulerian rather than Lagrangian measurements. Such approaches can provide good estimates of the instantaneous light field but do not necessarily reflect the doses of light experienced by particles and chemical species passively transported by physical processes within a lake or ocean surface‐layer. Here, a novel dual‐dye approach, used as a Lagrangian light dosimeter, was applied in Lake Superior's surface mixed layer to determine the importance of mixing in affecting the light dose experienced by particles passively mixed within is this layer. Two fluorescent dyes were deployed in the water column in a known ratio; one of the dyes (fluorescein) was sensitive to light exposure and one (rhodamine WT) was relatively photostable. The photochemistry of the dyes (i.e., the quantum yield of fluorescence bleaching at different wavelengths) and the role of matrix effects on dye response in a natural water system were explored with laboratory experiments. Decay equations for fluorescein and rhodamine WT fluorescence were determined from controlled irradiations of dye solutions with natural sunlight. The mixed‐layer light dose (from 460 nm to 510 nm) was then traced for two dye deployments in Lake Superior. The results show the importance of vertical mixing on the light dose within Lake Superior's surface layer on time scales of hours.
Abstract : The overall objective of this study is to develop models of radiative transfer for optically shallow waters with benthic substrates colonized by submerged aquatic vegetation (SAV). Such models will enable the quantitative prediction of upward spectral radiation from vegetated seabeds, permitting the use of optical remote sensing to search for submerged objects of anthropogenic origin and for rapid mapping of submarine resource distribution and abundance in coastal waters. These models will also have important applications for predicting irradiance levels within SAV canopies, a task necessary for accurate determination of light requirements and photosynthetic productivity of these ecologically important organisms.
The uptake of nitrate by phytoplankton is a central issue in biological oceanography due to its importance to primary production and vertical flux of biogenic carbon. Nitrate reductase catalyzes the first step of nitrate assimilation, the reduction of NO(3) to NO(2). A cytometric protocol to detect and quantify relative changes in nitrate reductase (NR) protein content of the marine centric diatom Skeletonema costatum is presented.Immunolabeling of NR protein was achieved with polyclonal antibodies raised against S.costatum NR. Antisera specific to a NR protein subunit and to a NR polypeptide sequence were compared, and cytometric results of NR protein abundance were related to Western analyses. Changes in cellular NR abundance and activity were followed during an upwelling simulation experiment in which S. costatum was exposed to a shift from ammonia to nitrate as major nitrogen source.NR protein could be detected in NO(3)-grown cells and at extremely low levels hardly discernible by Western Blot densiometry in NH(4)-grown cells. The protocol allowed observation of early stages of NR induction during an upwelling simulation. NR abundance increased after the nutrient shift to reach a new physiological "steady-state" 96 hrs later. NR activity exhibited diel variation with maxima at mid-day. NR abundance as estimated by both flow cytometry and Western analysis exhibited a hyperbolic relationship to NR activity. This pattern suggests post-translational activation of NR protein.The presented protocol allows the differentiation of NH(4)- versus NO(3)-grown algae as well as the monitoring of early stages in the induction of nitrate assimilatory capacities.
ABSTRACT Introduction: The Chesapeake Bay was once renowned for expansive meadows of submerged aquatic vegetation (SAV). However, only 10% of the original meadows survive. Future restoration efforts will be complicated by accelerating climate change, including physiological stressors such as a predicted mean temperature increase of 2–6°C and a 50–160% increase in CO 2 concentrations. Outcomes: As the Chesapeake Bay begins to exhibit characteristics of a subtropical estuary, summer heat waves will become more frequent and severe. Warming alone would eventually eliminate eelgrass ( Zostera marina) from the region. It will favor native heat-tolerant species such as widgeon grass ( Ruppia maritima ) while facilitating colonization by non-native seagrasses (e.g., Halodule spp.). Intensifying human activity will also fuel coastal zone acidification and the resulting high CO 2 /low pH conditions may benefit SAV via a “CO 2 fertilization effect.” Discussion: Acidification is known to offset the effects of thermal stress and may have similar effects in estuaries, assuming water clarity is sufficient to support CO 2 -stimulated photosynthesis and plants are not overgrown by epiphytes. However, coastal zone acidification is variable, driven mostly by local biological processes that may or may not always counterbalance the effects of regional warming. This precarious equipoise between two forces – thermal stress and acidification – will be critically important because it may ultimately determine the fate of cool-water plants such as Zostera marina in the Chesapeake Bay. Conclusion: The combined impacts of warming, coastal zone acidification, water clarity, and overgrowth of competing algae will determine the fate of SAV communities in rapidly changing temperate estuaries.
Temporal patterns of nutrient input into a Southern California kelp forest were measured using traditional hydrocast sampling coupled with high frequency temperature profiling. Patterns of nutrient input were related to growth rates of Macrocystis pyrifera located in an adjacent kelp forest. There were 2 distinct components to the pattern of nutrient availability. The long term, or seasonal, component was consistent with large-scale storm-induced mixing and horizontal advection during winter months. In addition, vertical motions of the thermocline, bringing nutrients into the kelp forest, occurred throughout the year with a frequency of about 2 per day and were strongest during the summer months. Weekly hydrocast sampling methods were inadequate for measuring these episodic events, and high frequency sampling was required to resolve the pattern of nutrient input accurately. Although measurable, nutrient input from vertical thermocline motion was inadequate to sustain maximum growth of Macrocystis pyrifera at 10m depth during the summer months. Thus, the major component of nutrient input came during the winter. These results indicate that nitrate limitation of M. pyrifera is a likely cause of reduced summer growth. Further, high frequency sampling is necessary to predict nutrient availability in nearshore ecosystems dominated by benthic macrophytes where the pattern of nutrient input is dominated by episodic events of short duration.
