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The distribution of zooplankton communities from the source to the mouth of a lowland water course, the Scheldt, was used to test some concepts on the longitudinal distribution of organisms along both estuarine (Remane's diversity concept) and riverine reaches (River Continuum Concept (RCC), Riverine Ecosystem Synthesis (RES)). Substantial zooplankton densities were found in both April and June samples all along the water course, and not only in the lower reaches as suggested by the RCC. Based on the zooplankton communities, and using hierarchical classifications and Generalized Linear Model, several successive zones influenced by different physical and chemical variables and trophic conditions could be distinguished along the Scheldt, in agreement with the RES model: the non-tidal riverine, the tidal freshwater, the tidal salinity gradient, and the marine zones. Over the entire freshwater reach, rotifers were numerically dominant during both samplings; crustaceans became more abundant in June than in April, specifically in the downstream reaches. Highest total zooplankton abundance and diversity were found in the tidal freshwater reach, lowest in the brackish water reach, in accordance with Remane's concept. An influence of the tributaries on zooplankton communities is suggested, directly by import or dilution of zooplankton communities or through changing environmental conditions. [ABSTRACT FROM AUTHOR]

In eutrophic areas, green macroalgae are frequently and for long periods arranged in mats, resulting in a steep light gradient. This study investigates the effect of this gradient on physiological characteristics [tissue nitrogen content, maximal photosynthetic efficiency (Fv/Fm), glutathione levels and redox ratio, absorbance and absorption spectra] of the green macroalga Ulva spp. Mats were sampled during the build-up (June), stationary (July), and decomposing (September) phases of a macroalgal bloom in the Veerse Meer, a eutrophic brackish (salinity 15–20 psu) lake in the southwest Netherlands. Water samples were taken for nutrient analyses. At all three sampling dates, the mats were composed almost entirely of Ulva spp.; in September the mats were in decay and covered with silt and epiphytes. In June and July, total dissolved inorganic nitrogen concentration (DIN) of the water within the mat was significantly higher than outside the mat. Pronounced vertical differences were found in tissue N, Fv/Fm values, total glutathione levels, glutathione redox ratios, and absorbance. In June and July, tissue N decreased from over 2.2% dry weight (DW; N-sufficient) in the bottom layers to around 1% DW (minimum level for survival) in the top layers. Wide-band absorption increased with depth in the mat and throughout the season, probably due to higher Chl a and b and lutein contents. The shape of the absorption spectrum was similar for all layers. The absorption of the silt/epiphyte film on the top Ulva layer was highest; its absorption spectrum (high absorption in the 500–560 nm range) indicates that the film on the top layers of the macroalgal mats mainly consisted of diatoms. In June, Fv/Fm and the glutathione redox ratio of the algae increased with depth in the layer, while total glutathione decreased. Low Fv/Fm values in the bottom and middle layers in September reflect the bad condition of the algae; the mats were largely decaying. It is concluded that multiple growth-limiting gradients occur in macroalgal mats: upper layers suffer from nitrogen limitation and photoinhibition while bottom layers are light limited. The algae in the mat acclimatize to low light conditions by increasing their absorption through increased pigment contents and by higher photosynthetic efficiency during the build-up and stationary period. This study qualifies the glutathione redox ratio as a promising candidate for stress indicator in macroalgae and provides suggestions for its further development. [ABSTRACT FROM AUTHOR]

In late summer and autumn, before the vertical circulation reaches the thermocline, the phytoplankton population of Lake Zürich is dominated by the red-coloured filamentous cyanobacterium Planktothrix rubescens, which stratifies in the metalimnion at depths close to the photosynthetic compensation point. The filament volume concentration reached a maximum of 12 cm3 m-3; the depth of the maximum varied from 10.5 to 12.5 m. Changes in the depth distribution were attributed to a combination of (1) seiche movements, which raised or lowered the thermocline by up to 2 m over 36 h, and (2) flotation by the buoyant filaments relative to the isotherms, by up 0.4 m d-1. These changes caused a 2-fold change in insolation at the Planktothrix peak. Estimates were made of the daily integral of photosynthetic O2-production, ΣΣ(NP), by the population of P. rubescens over a period of four cloudless days. The estimates were calculated from measurements of surface irradiance (at 5-min intervals), vertical light attenuation, temperature, filament volume concentration and the photosynthesis/irradiance ( P/I) curves of filaments concentrated from the metalimnion. Despite the similar, high insolation on each of the four days, the calculated values of ΣΣ(NP) varied from 9 to 53 mmol m-2 d-1, owing to the changing depth distribution of the filaments. Measurements of P/I curves of lakewater samples incubated at a depth of 11 m showed changes in the photosynthetic coefficients during the day. These also generated large changes in calculated values of ΣΣ(NP). The computer spreadsheet used to calculate ΣΣ(NP) was modified to incorporate time-based changes in the photosynthetic coefficients and vertical distribution of the organism. These refinements provide a more accurate description of photosynthesis by the deep-living P. rubescens, which adjusts its position by buoyancy regulation to exploit the light field in the metalimnion, where it outcompetes other phytoplankton. [ABSTRACT FROM AUTHOR]