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The annual cycle of oxygen and nitrogen flux, denitrification and microphytobenthic variables (primary production, biomass, composition and calculated N demand) were studied for 2 shallow-water microtidal sediment sites in NE Kattegat, 1 sandy and 1 silty, by incubating undisturbed sediment in the laboratory in light and darkness. Both sites (2 stations within each) were characterised by low concentrations of inorganic nitrogen (IN), in the overlying water during summer (NO3 generally < l pM), with winter concentrations of 5 and 30 pM for the sandy and silty site, respectively. Through the activity of microphytobenthos, the sediment systems appeared to be net autotrophic during most of the year. Net oxygen production varied between -400 and 6600 pm01 m-? h-', being highest during the warmest season. Although the composition of the microphytobenthos depended on the sediment type, it did not have a crucial effect on the magnitude of the microphytobenthic biomass or function. The .temporal pattern of the function of the microalgal community, on the other hand, was significantly influenced by the sediment type; sandy sediment exhibited a smooth seasonality, controlled mainly by temperature and light, while the silty microtidal sediment was also controlled by stochastic events, such as sediment resuspension. Microphytobenthos had a significant effect on the IN flux, the clearest effect being found for NH4. Total denitrification (isotope-pairing technique) generally varied between < l and 40 pm01 m-2 h-', being dominated by nitrification-coupled denitrification (D,,), and being 1 order of magnitude higher at the silty site. Microphytobenthic activity generally inhibited denitrification in the low-N areas in this study. The results suggest that the microphytobenthos functions as a major control throughout the annual cycle, by forming communities that are net photoautotrophic throughout the year, and by significantly influencing both the IN flux and denitrification rates. Sandy sediment appeared to function as an IN sink during winter and early spring, while no clear seasonal pattern was found for silty sediment. Calculated N demand of the microphytobenthos far exceeded the measured sediment net uptake of N, supporting the idea that sandy systems in low-nitrogen areas can be highly productive through a closed recycling of N. The ratio between calculated microphytobenthic N demand and measured denitrification rates suggests that denitrification has a minor role as a N sink, particularly in sandy, cold-climate microtidal sediments. MARINE ECOLOGY PROGRESS SERIES Mar Ecol Prog Ser

Despite a large body of research concerning mercury (Hg) in birds, no single tissue has been used consistently to assess Hg exposure, and this has hampered comparisons across studies. We evaluated the relationships of Hg concentrations among tissues in four species of waterbirds (American avocets [Recurvirostra americana], black-necked stilts [Himantopus mexicanus], Caspian terns [Hydroprogne caspia; formerly Sterna caspia], and Forster's terns [Sterna forsteri]) and across three life stages (prebreeding adults, breeding adults, and chicks) in San Francisco Bay, California, USA. Across species and life stages, Hg concentrations (least square mean +/- standard error) were highest in head feathers (6.45 +/- 0.31 microg/g dry wt) and breast feathers (5.76 +/- 0.28 microg/g dry wt), followed by kidney (4.54 +/- 0.22 microg/g dry wt), liver (4.43 +/- 0.21 microg/g dry wt), blood (3.10 +/- 0.15 microg/g dry wt), and muscle (1.67 +/- 0.08 microg/g dry wt). Relative Hg distribution among tissues, however, differed by species and life stage. Mercury concentrations were highly correlated among internal tissues (r2 > or = 0.89). Conversely, the relationships between Hg in feathers and internal tissues were substantially weaker (r2 < or = 0.42). Regression slopes sometimes differed among species and life stages, indicating that care must be used when predicting Hg concentrations in one tissue based on those in another. However, we found good agreement between predictions made using a general tissue-prediction equation and more specific equations developed for each species and life stage. Finally, our results suggest that blood is an excellent, nonlethal predictor of Hg concentrations in internal tissues but that feathers are relatively poor indicators of Hg concentrations in internal tissues.

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