Food and feeding habits of juvenile flounder Platichthys flesus (L.), abd turbot Scophthalmus maximus L. in the åland archipelago, northern Baltic Sea

The food choice of juvenile flounder (Platichthys flesus) and turbot (Scophthalmus maximus) was studied in the northern Baltic Sea during the years 1988, 1989, 1994 and 1995. The diet included organisms from 30 species/taxa in flounder (n = 306) and 10 species/taxa in turbot (n = 41). Flounder 45 mm mainly consumed meiofauna (dominating taxon: Harpacticoida, Copepoda) and larger fish (46–101 mm) consumed macrofauna (dominating taxa: Oligochaeta, Amphipoda and Chironomidae). In terms of biomass, macrofauna dominated for all sizes of flounders, and meiofauna was important only for the smallest fish. A strong seasonal variation could be detected in the diet. In spring, macrofauna dominated for all size classes of fish (only fish > 30 mm were caught in spring), while in summer and autumn meiofauna dominated the diets for fish 45 mm in size. Juvenile turbot (22–88 mm) consumed macrofauna and small fish. Turbot 30 mm consumed mainly amphipods, while > 30 mm turbot consumed mysid shrimps, amphipods and fish.The ontogenetic shift from meio- to macrofauna-sized prey in flounders occurs at a larger fish size in the northern Baltic Sea than reported in other areas, possibly depending on the increased relative importance of meiofauna in the northern Baltic. The seasonal variation in the diet could be due to seasonally changing abundances in the zoobenthos, or for the small fish (1-group, spring), to switching from meio- to macrofauna in order to optimize their energy gain. The 0-group flounders consumed meiofauna for a long period, possibly due to a learning-process or simply due to easy availability of meiofauna. Turbot has a much larger mouth gap than flounders, thus allowing them to consume macrofauna from the beginning of their benthic life.     

Gravestone geology

Cemeteries and graveyards not only serve as a peaceful place to commemorate the dead. For geologists—whether amateur, student or professional—almost any urban cemetery provides a valuable opportunity to carry out scientific field work at leisure, right on the doorstep, and at no cost. Cemeteries offer an excellent opportunity to examine and study a wide range of stone types in a small space. They also serve as a useful field area for studying rock weathering.     

A SEARCH FOR VECTOR MAGNETIC FIELD VARIATIONS ASSOCIATED WITH THE M-CLASS FLARES OF 10 JUNE 1991 IN AR 6659

A careful analysis of a 6-hour time sequence of vector magnetograms of AR 6659, observed on 10 June 1991 with the MSFC vector magnetograph, has revealed only minor changes in the vector magnetic field azimuths in the vicinity of two M-class flares, and the association of these changes with the flares is not unambiguous. In this paper we present our analysis of the data which includes comparison of vector magnetograms prior to and during the flares, calculation of distributions of the r.m.s. variation of the azimuth at each pixel in the field of view of the active region, and examination of the variation with time of the azimuths along the flaring neutral lines and at every pixel covered by the main flare emissions as observed with the H telescope coaligned with the vector magnetograph.     

Effect of hydrologic management on marsh surface sediment deposition in coastal Louisiana

High rates of coastal land loss in Louisiana have prompted efforts to maintain or restore coastal wetland habitats, and structural management of marsh hydrology is one of a number of approaches that has been adopted. The aim of this study was to determine the effect of hydrologic management measures on marsh-surface sediment deposition in the Mississippi deltaic plain. Four impoundments, ranging in size from 50 ha to 177 ha and similarly sized control sites were included in the study. At each site, marsh-surface sediment traps were collected approximately biweekly for 3.5 yr to measure changes in sediment deposition. There was no significant difference in sediment deposition between any of the impoundment and control sites during the premanagement period. The results show no significant difference in marsh-surface sediment deposition between management year 1992–1993 and management year 1994–1995, but management year 1993–1994 had significantly lower sediment deposition than either the first or the last year of the study. Management year 1992–1993 showed the highest sediment deposition when it was assessed across all basins, areas, and sites, and this high rate of deposition is accounted for by the impact of Hurricane Andrew. For all management years there were significant differences between impoundment and control sites, with control sites showing higher rates of marsh surface sediment deposition than impoundments. No clear pattern was identified concerning the influence of various types of hydrologic management on rates of sediment deposition. This reduction in sediment deposition indicated that the areas under management in this study are receiving insufficient inorganic sediment deposition to keep pace with sea-level rise.     

Carbon dioxide concentration and atmospheric flux in the Hudson River

We made direct measurements of the partial pressure of CO₂ (P_{CO 2}) in the tidal-freshwater portion of the Hudson River Estuary over a 3.5-yr period. At all times the Hudson was supersaturated in CO₂ with respect to the atmosphere. P_{CO 2} in surface water averaged 1125±403 (SD) μatm while the atmosphere averaged 416±68 μatm. Weekly samples at a single, mid-river station showed a pronounced and reproducible seasonal cycle with highest values (～2000 μatm) in mid-to-late summer, and lowest values (～500 μatm) generally in late winter. Samples taken along the length of the 190-km section of river showed a general decline in CO₂ from north to south. This decline was most pronounced in summer and very slight in spring. Diel and vertical variation were small relative to the standing stock of CO₂. Over six diel cycles, all taken during the algal growing season, the mean range was 300±114 μatm. CO₂ tended to increase slightly with depth, but the gradient was small, about 0.5 μmol m^?1, or an increase of 190 μatm from top to within 1 m of the bottom. For a large subset of the samples (n=452) we also calculated CO₂ from measurements of pH and total DIC. Calculated and measured values of CO₂ were in reasonably good agreement and a regression of calculated versus measured values had a slope of 0.85±0.04 and an r₂ of 0.60. Combining our measurements with recent experimental studies of gas exchange in the Hudson, we estimate that the Hudson releases CO₂ at a rate of 70–162 g C m^?2 yr^?1 from the river to the atmosphere.     

Filtration of Hudson River water by the zebra mussel (Dreissena polymorpha)

Zebra mussels (Dreissena polymorpha) graze on phytoplankton, and decreased phytoplankton concentrations have been associated with zebra mussels in lakes. It is not known, however, how the zebra mussel will affect phytoplankton in turbid systems such as rivers and the freshwater portions of estuaries. To determine whether zebra mussels can effectively remove phytoplankton in these turbid systems, and to determine what components of the suspended material are removed and at what rates, we conducted a series of grazing and size-selection experiments using ambient Hudson River water and its natural phytoplankton community. Zebra mussels removed both phytoplankton and total suspended weight (TSW) at comparable rates (～115 ml mussel^?1 h^?1). Variation in filtration rates were not correlated with TSW or chlorophylla (chla) concentration, and did not appear to depend on relative proportions of either component. Mussels removed particles with approximately equal efficiency in all particle size classes measured (0.4 μm to >40 μm). Zebra mussels appear to remove Hudson River phytoplankton effectively in the presence of suspended sediment and do so at rapid rates. Based on our measurements and unpublished estimates of the size of the population, zebra mussels filter a volume equivalent to the entire volume of the tidal freshwater portion of the Hudson River about every 2 d.