Winds and the distribution of nearshore phytoplankton in a stratified lake.

The distribution of phytoplankton in lakes is notoriously patchy and dynamic, but wind-driven currents and algal buoyancy/motility are thought to determine where algae accumulate. In this study, nearshore phytoplankton were sampled from different parts of a lake basin twice a day for 4-5 consecutive days, in the spring and in late summer, to test whether short-term changes in phytoplankton biomass and community composition can be predicted from wind-driven currents. On windy days, phytoplankton biomass was higher at downwind than at upwind nearshore sites, and the magnitude of this difference increased linearly with increasing wind speed. However, contrary to the generally assumed downwind phytoplankton aggregations, these differences were mostly due to upwelling activity and the dilution of phytoplankton at upwind nearshore sites. The distribution of individual taxa was also related to wind speed, but only during late stratification (except for cryptophytes), and these relationships were consistent with the buoyancy and motility of each group. On windy days, large diatoms and cyanobacteria concentrated upwind, neutrally buoyant taxa (green algae, small diatoms) were homogeneously distributed, and motile taxa (cryptophytes, chrysophytes, dinoflagellates) concentrated downwind. Predictable differences in the biomass and composition of phytoplankton communities could affect the efficiency of trophic transfers in nearshore areas.

Phosphorus sorption experiments and the potential for internal phosphorus loading in littoral areas of a stratified lake.

The exchange of phosphorus (P) during the resuspension of sediments into shallow (oxic) waters of deep stratified lakes is regulated by equilibrium dynamics. In this study, we compared the P-sorption characteristics of sediments from 17 shallow and deep littoral sites in an oligo-mesotrophic lake. Zero Equilibrium P Concentration (EPC(0)) ranged from 0.2 to 5 microgPL(-1). EPC(0) did not vary with sediment characteristics, but increased with increasing sediment-to-water ratios (SWR). Buffering capacity also increased with increasing SWR up to 1 gL(-1), at which point P concentrations were buffered almost perfectly. Therefore, internal P loading in littoral areas may depend primarily on the intensity and duration of sediment resuspension instead of sediment composition, and is expected to be spatially and temporally patchy. Maximum P-sorption capacity (S(max)) varied with chemical composition of the sediments, but was generally low, indicating a limited capacity of littoral sediments to retain external inputs of P.

Energy-dense snack food intake in adolescence: longitudinal relationship to weight and fatness.

OBJECTIVE: The longitudinal relationship between the consumption of energy-dense snack (EDS) foods and relative weight change during adolescence is uncertain. Using data from the Massachusetts Institute of Technology Growth and Development Study, the current analysis was undertaken to examine the longitudinal relationship of EDS food intake with relative weight status and percentage body fat and to examine how EDS food consumption is related to television viewing. RESEARCH METHODS AND PROCEDURES: One hundred ninety-six nonobese premenarcheal girls 8 to 12 years old were enrolled between 1990 and 1993 and followed until 4 years after menarche. At each annual follow-up visit, data were collected on percentage body fat (%BF), BMI z score, and dietary intake. Categories of EDS foods considered were baked goods, ice cream, chips, sugar-sweetened soda, and candy. RESULTS: At study entry, girls had a mean +/- SD BMI z score of -0.27 +/- 0.89, consumed 2.3 +/- 1.7 servings of EDS foods per day, and consumed 15.7 +/- 8.1% of daily calories from EDS foods. Linear mixed effects modeling indicated no relationship between BMI z score or %BF and total EDS food consumption. Soda was the only EDS food that was significantly related to BMI z score over the 10-year study period, but it was not related to %BF. In addition, a significant, positive relationship was observed between EDS food consumption and television viewing. DISCUSSION: In this cohort of initially nonobese girls, overall EDS food consumption does not seem to influence weight status or fatness change over the adolescent period.

Longitudinal changes in the accuracy of reported energy intake in girls 10-15 y of age.

BACKGROUND: Dietary records are often used to estimate individual energy needs and population energy requirements. However, significant underreporting of total energy intake (EI) has been found when EI is compared with total energy expenditure (EE) measured by doubly labeled water. OBJECTIVE: This study aimed to determine whether the accuracy of reported EI decreases from middle childhood to adolescence. DESIGN: In this longitudinal study of 26 healthy girls, EI and EE were measured at ages 10, 12, and 15 y. Accuracy of reported EI (EI/EE x 100%) was calculated at each age. At study entry, girls had a mean (+/- SD) body mass index (in kg/m(2)) of 16.8 +/- 1.9 and percentage body fat of 24.0 +/- 4.6%. Measurements of EI were a 7-d dietary record and those of EE were by doubly labeled water. RESULTS: As they got older, girls tended to report EI less accurately: the average accuracy was 88 +/- 13% at age 10 y, 77 +/- 21% at age 12 y, and 68 +/- 17% at age 15 y. The declines in reporting accuracy from age 10 y to age 12 y and from age 10 y to age 15 y were statistically significant (P = 0.03 and P = 0.001, respectively). Reporting accuracy also declined from age 12 to age 15 y but not significantly. When percentage body fat was added to the model, results were essentially unchanged. CONCLUSION: Because of the decline in EI reporting accuracy with age, the use of EI data obtained from dietary records in adolescent girls will result in substantial underestimation of energy needs.

Zooplankton community size structure and taxonomic composition affects size-selective grazing in natural communities.

The body size of an individual zooplankton is well related to its grazing rate and to the range of particle sizes it can ingest, and since cladocerans and copepods feed differently, they follow different relationships. Based on these general patterns in individual organisms, we tested whether the size structure and taxonomic composition of more complex natural zooplankton communities are related to their in situ grazing rate and to the range of algal sizes they graze. We compared community grazing rates on individual algal taxa in two communities dominated by small cladocerans, three communities dominated by large cladocerans and three copepod-dominated communities. Small algae were usually grazed most intensively, but grazing rates were poorly related to algal size alone. The range in size of grazed algae increased with increasing mean zooplankton body size, but differed systematically with their taxonomic composition. Communities dominated by Ceriodaphnia or Holopedium grazed a narrower size range of algae [maximum greatest axial length dimension (GALD)=16-36 µm)] than communities with large biomasses of Bosmina or Daphnia (maximum GALD=28-78 µm). Copepod-dominated communities followed the same general relationship as cladocerans. Daphnia-dominated communities grazed the broadest range of algal sizes, and their total grazing rates were up to 2.4 times their grazing rates on small (<35 µm) "highly edible" algae, a difference of similar magnitude to those found in successful trophic cascade biomanipulations.