Decadal Declines of Mercury in Adult Bluefish (1972-2011) from the Mid-Atlantic Coast of the U.S.A.

Concentrations of total mercury were measured in muscle of adult bluefish (Pomatomus saltatrix) collected in 2011 off North Carolina and compared with similar measurements made in 1972. Concentrations of mercury decreased by 43% in the fish between the two time periods, with an average rate of decline of about 10% per decade. This reduction is similar to estimated reductions of mercury observed in atmospheric deposition, riverine input, seawater, freshwater lakes, and freshwater fish across northern North America. Eight other studies between 1973 and 2007 confirm the decrease in mercury levels in bluefish captured in the Mid-Atlantic Bight. These findings imply that (1) reductions in the release of mercury across northern North America were reflected rather quickly (decades) in the decline of mercury in adult bluefish; (2) marine predatory fish may have been contaminated by anthropogenic sources of mercury for over 100 years; and (3) if bluefish are surrogates for other predators in the Mid-Atlantic Bight, then a reduction in the intake of mercury by the fish-consuming public has occurred. Finally, with global emissions of mercury continuing to increase, especially from Asia, it is important that long-term monitoring programs be conducted for mercury in marine fish of economic importance.

Photosynthetic maximum quantum yield increases are an essential component of the Southern Ocean phytoplankton response to iron.

It is well established that an increase in iron supply causes an increase in total oceanic primary production in many regions, but the physiological mechanism driving the observed increases has not been clearly identified. The Southern Ocean iron enrichment experiment, an iron fertilization experiment in the waters closest to Antarctica, resulted in a 9-fold increase in chlorophyll (Chl) concentration and a 5-fold increase in integrated primary production. Upon iron addition, the maximum quantum yield of photosynthesis (phi(m)) rapidly doubled, from 0.011 to 0.025 mol C.mol quanta(-1). Paradoxically, this increase in light-limited productivity was not accompanied by a significant increase in light-saturated productivity (P(max)(b)). P(max)(b), maximum Chl normalized productivity, was 1.34 mg C.mg Chl(-1).h(-1) outside and 1.49 mg C.mg Chl(-1).h(-1) inside the iron-enriched patch. The importance of phi(m) as compared with P(max)(b) in controlling the biological response to iron addition has vast implications for understanding the ecological response to iron. We show that an iron-driven increase in phi(m) is the proximate physiological mechanism affected by iron addition and can account for most of the increases in primary production. The relative importance of phi(m) over P(max)(b) in this iron-fertilized bloom highlights the limitations of often-used primary productivity algorithms that are driven by estimates of P(max)(b) but largely ignore variability in phi(m) and light-limited productivity. To use primary productivity models that include variability in iron supply in prediction or forecasting, the variability of light-limited productivity must be resolved.

The effect of advection on the nutrient reservoir in the North Atlantic subtropical gyre.

Though critically important in sustaining the ocean's biological pump, the cycling of nutrients in the subtropical gyres is poorly understood. The supply of nutrients to the sunlit surface layer of the ocean has traditionally been attributed solely to vertical processes. However, horizontal advection may also be important in establishing the availability of nutrients. Here we show that the production and advection of North Atlantic Subtropical Mode Water introduces spatial and temporal variability in the subsurface nutrient reservoir beneath the North Atlantic subtropical gyre. As the mode water is formed, its nutrients are depleted by biological utilization. When the depleted water mass is exported to the gyre, it injects a wedge of low-nutrient water into the upper layers of the ocean. Contrary to intuition, cold winters that promote deep convective mixing and vigorous mode water formation may diminish downstream primary productivity by altering the subsurface delivery of nutrients.

Southern Ocean iron enrichment experiment: carbon cycling in high- and low-Si waters.

The availability of iron is known to exert a controlling influence on biological productivity in surface waters over large areas of the ocean and may have been an important factor in the variation of the concentration of atmospheric carbon dioxide over glacial cycles. The effect of iron in the Southern Ocean is particularly important because of its large area and abundant nitrate, yet iron-enhanced growth of phytoplankton may be differentially expressed between waters with high silicic acid in the south and low silicic acid in the north, where diatom growth may be limited by both silicic acid and iron. Two mesoscale experiments, designed to investigate the effects of iron enrichment in regions with high and low concentrations of silicic acid, were performed in the Southern Ocean. These experiments demonstrate iron's pivotal role in controlling carbon uptake and regulating atmospheric partial pressure of carbon dioxide.

The low-light reduction in the quantum yield of photosynthesis: potential errors and biases when calculating the maximum quantum yield.

Photosynthesis-irradiance (P-E) curves are widely used to describe photosynthetic efficiency and potential. Contemporary models assume maximal photosynthetic quantum yield (phi) at low irradiances. But P-E observations made with both oxygen evolution and carbon uptake techniques show that this is not always the case. Using new and published data in conjunction with modeling exercises, we demonstrate that regardless of the mechanism there can be reductions in phi at low irradiances that are not readily observable using conventional P-E analyses. We also show that analytical errors, such as inaccurate estimation of dark oxygen consumption or carbon uptake, can markedly affect the structure of phi-E curves with negligible effect on P-E curve structure. Whether from respiration ;corrections' or other mechanisms, these deviations in phi at low light levels from the maximum quantum yield of photosynthesis (phi(max)) can lead to significant errors (> 50%) in the estimation of the linear portion of the P-E curve and ultimately phi(max). Non-linear models of P-E, such as the rectangular hyperbola, quadratic, exponential and hyperbolic tangent that are commonly used to estimate the initial slope (alpha) of the P-E curve assume that phi is maximal at low light levels and therefore can err in the estimation of phi(max) when phi is reduced at low light levels. Using a diverse data set of 622 P-E curves with a total of 7623 points, we show that although model skills are high (r (2) = 0.96 +/- 0.05, 0.97 +/- 0.04, 0.97 +/- 0.04 and 0.97 +/- 0.04, respectively), a large fraction of the model-predicted phi(max) differ by greater than 10% from true phi(max) values (91%, 50%, 82% and 46%, respectively). Data from these observations and modeling exercises lead us to suggest that phi(max) be determined by directly estimating the true maximum of a phi-E curve rather than using the more conventional methodology employing the initial slope of the P-E curve.