Effects of riparian leaf dynamics on periphyton photosynthesis and light utilisation efficiency

1. Streambed light regimes change dramatically when riparian trees gain leaves in spring and lose them in autumn. This study examined the effect of these changes on periphyton photosynthetic characteristics, primary production, and light utilisation efficiency in two eastern Tennessee streams. 2. Photosynthesis–irradiance responses were measured at intervals covering leaf emergence and abscission in spring and autumn. Photosynthetic efficiency (α^chl) increased with declining streambed irradiances during spring leaf emergence, but returned to pre‐emergence values after autumn leaf fall. The onset of photosaturation (I_k) displayed the opposite pattern, decreasing during leaf emergence and increasing after leaf fall. Both α^chl and I_k were closely associated (P < 0.01) with daily integrated streambed irradiance, as were periphyton carotenoids. Internal shading by photoprotective carotenoids is hypothesised to account for lower α^chl when streambed irradiances are high. 3. An in situ shading experiment confirmed that the temporal changes observed in periphyton photosynthetic characteristics and carotenoids were primarily the result of changing light levels and not other environmental factors (e.g. nutrients, temperature). 4. Daily chlorophyll‐specific primary production (PP^chl) was calculated with P–I models and recorded streambed irradiances. In both streams, PP^chl was the highest in early spring when trees were leafless, and then declined markedly as leaves emerged, reaching a minimum in summer. PP^chl increased after leaf abscission, but was still lower than it was in early spring, when the sun was higher and daylength was longer. A hyperbolic tangent equation fit to PP^chl and daily integrated irradiance (r²=0. 85) suggested that primary production was light saturated at 4–8 mol m^–2 d^–1. 5. Light utilisation efficiency (Ψ) increased 10‐fold during leaf emergence. Photosaturation at high irradiances and photoacclimation at lower irradiances were responsible for a negative hyperbolic relationship between Ψ and daily integrated irradiance.     

Dynamic status of primary production in lake yunoko,a small eutrophic subalpine lake in central Japan

Investigation was made of the primary productivity of Lake Yunoko. The concentrations of PO₄-P and NH₃-N were highest in summer. The total amount of chlorophylla in the entire water column varied from 45 mg/m² to 405 mg/m² during the year with the maximum values occurring in late spring and in December. These values are very high compared with those reported for other lakes in the same district. The standing crop of phytoplankton as organic carbon was approximately 50∼70% of the total particulate organic carbon in the lake with values as high as 80∼90% being obtained in the spring and autumn. The rate of phytosynthesis under saturated light conditions was 3.7 mg C/mg chla/hr in the case of phytoplankton of the sun type and 1.2∼1.6 mg C/mg chla/hr in those of the shade type. The daily gross primary production in the lake varied from a low of 134 mg C/m² to a high of 1,003 mg C/m² during the year, the mean value being 372 mg C/m². The total amount of daily microbial respiration varied from 177 mg C/m² in winter to 1,476 mg C/m² in summer. Thus, the daily balance of production and consumption of organic matter in the lake itself was concluded to be negative. The main factor contributing to the rectification of this imbalance of material budget seems to be the leaf litter coming from the forest surrounding the lake. In conclusion, Lake Yunoko is a very productive and at the same time, fairly heterotrophic lake. Special Project Research supported by the Ministry of Education on Dynamic State of Biosphere. This study was carried out as part of JIBP-PF.     

The determination of desirable and nuisance plant levels in streams

To cope with luxuriant plant growth in the streams of southern Ontario, an approach to determine the desirable and nuisance plant levels was proposed.With a more intensive analysis of the assimilation efficiency of plant communities, which includes the fractionation of community respiration as well as compensation for the plant self-shading effect, the plant biomass of all species component expressed as chlorophyll a can be estimated from the modified growth equation, where B is the biomass, P_max is the photosynthetic growth rate at light saturation, A is the assimilation number and S is the plant shading coefficient. Once comparable biomass values are available, a desirable plant level relative to specified environmental standards can be determined.For instance, to meet the minimum dissolved oxygen criteria of 5 ppm, the desirable and the nuisance crop levels in the North Thames River, Ontario, were found to be 0.15 and 0.75 g chla/m² respectively.     

Microcrustacean community structure and biomass in marsh and lake habitats of the Okefenokee Swamp: seasonal dynamics and responses to resource manipulations

Microcrustacean community and biomass dynamics were studied for two years in a Nymphaea-Eriocaulon macrophyte marsh and a nearby shallow lake which lacked macrophytes in the Okefenokee Swamp. In this blackwater, acidic wetland, microcrustacean diversity and biomass were similar to other circumneutral lakes and littoral areas, contributing to a productive fish assemblage. In the lake, the annual biomass pattern (15–1627 μg 1⁻¹) was unimodal and was dominated by the crustaceans Diaptomus sinuatus and Eubosmina tubicen. Rotifers were occasionally important, constituting up to 55% of total biomass. Over the long term, mean annual biomass in this post-drought study are higher than in pre-drought years. In the marsh, biomass (11–777 μg 1⁻¹) fluctuated biomodally with late winter depressions corresponding to low temperatures and midsummer declines indicative of increasing fish predation. Summer dominance shifted between years from Macrothricidae in 1982 to Sididae in 1983. Variation in biomass correlated most strongly with algal chlorophyll in the marsh and with bacterial density in the lake. In field enclosure experiments in which primary production was reduced by shading, microcrustacean responses varied between lake and marsh habitats and with season. Lake zooplankton were consistently suppressed by reduced algal resources in winter, spring and summer experiments, with greatest responses to shading in the summer. Marsh microcrustacea were most affected in the winter experiment and became less sensitive to manipulated resource levels in spring and summer. Decoupling of these consumers from autotrophic resources in the marsh, but not in the lake, coincides with times of high macrophyte turnover and warming temperatures which promote the conversion of detritus into heterotrophic resources such as bacteria. The conflict between interactions implied by the experimental approach vs statistical criteria emphasizes a need to interpret resource dependence from seasonal dynamics of field populations with caution.     

BIOMASS,PRODUCTION, AND LITTERFALL IN THE COASTAL SAGE SCRUB OF SOUTHERN CALIFORNIA

A 22-year-old stand of coastal sage scrub in the coastal mountains of southern California had a peak standing aboveground biomass of 1,417 g/m², determined by dimension analysis. Annual aboveground net primary production was 255 g/m²/yr, determined by monthly twig harvests of dominant species and the clipping production of subordinate species. The stand was codominated by two drought-deciduous species, Salvia leucophylla and Artemisia californica, which together comprised 81% of the biomass. Annual litterfall was measured at 194 g/m²/yr. These biomass, production, and litterfall values are less than those measured in most evergreen chaparral communities in California. Seasonally, the two dominant shrubs began aboveground production in the winter soon after the first rains and continued growth for six months until early summer. A massive leaf fall occurred in May–June as the summer drought began, but twig and inflorescence production for both species continued at a high rate into the summer months. Salvia leucophylla had two shoot types: 1) an early spring canopy shoot that elongated rapidly, produced the inflorescence, and died in mid-summer; and 2) a short side shoot produced in late spring with small dense leaves that were retained during the summer drought and early winter. Artemisia californica produced a single cohort of twigs in the early spring, most of which carried inflorescences by late summer.     

Phytoplankton biomass and production in shelf waters off NW Spain: spatial and seasonal variability in relation to upwelling

Summary Chlorophyll-a and primary production on the euphotic zone of the N-NW Spanish shelf were studied at 125 stations between 1984 and 1992. Three geographic areas (Cantabrian Sea, Rías Altas and Was Baixas), three bathymetric ranges (20 to 60 m, 60 to 150 m and stations deeper than 200 m), and four oceanographic stages (spring and autumn blooms, summer upwelling, summer stratification and winter mixing) were considered. One of the major sources of variability of chlorophyll and production data was season. Bloom and summer upwelling stages have equivalent mean and maximum values. Average chlorophyll-a concentrations approximately doubled in every step of the increasing productivity sequence: winter mixing — summer stratification — high productivity (upwelling and bloom) stages. Average primary production rates increased only 60% in the described sequence. Mean (± sd) values of chlorophyll-a and primary production rates during the high productivity stages were 59.7 ± 39.5 mg Chl-a m^–2 and 86.9 ± 44.0 mg C m^–2 h^–1, respectively. Significant differences in both chlorophyll and primary production resulted between geographic areas in most stages. Only 27 stations showed the effects of the summer upwelling that affected coastal areas in the Cantabrian Sea and Rías Baixas shelf, but also shelf-break stations in the Rías Altas area. The Rías Baixas area had lower chlorophyll than both the Rías Altas and the Cantabrian Sea areas during spring and autumn blooms, but higher during summer upwelling events. On the contrary, primary production rates were higher in the Rías Baixas area during blooms in spring and autumn. Mid-shelf areas showed the highest chlorophyll concentrations during high productivity stages, probably due to the existence of frontal zones in all geographic areas considered. The estimated phytoplankton growth rates were comparable to those of other coastal upwelling systems, with average values lower than the maximum potential growth rates. Doubling rates for upwelling and stratification stages in the northern and Rías Altas shelf areas were equivalent, despite larger biomass accumulations during upwelling events. Low turnover rates of the existing biomass in the Rías Baixas shelf in upwelling stages suggests that the accumulation of phytoplankton was due mainly to the export from the highly productive rías, while the contribution of in situ production to these accumulations was relatively lower.     

Annual and seasonal changes in fine root biomass of a Quercus ilex L. forest

The biomass, production and mortality of fine roots (roots with diameter <2.5 mm) were studied in a typical Mediterranean holm oak (Quercus ilex L.) forest in NE Spain using the minirhizotron methodology. A total of 1212 roots were monitored between June of 1994 and March of 1997. Mean annual fine root biomass in the holm oak forest of Prades was 71±8 g m^–2 yr^–1. Mean annual production for the period analysed was 260+11 g m^–2 yr^–1. Mortality was similar to production, with a mean value of 253±3 g m^–2 yr^–1. Seasonal fine root biomass presented a cyclic behaviour, with higher values in autumn and winter and lower in spring and summer. Production was highest in winter, and mortality in spring. In summer, production and mortality values were the lowest for the year. Production values in autumn and spring were very similar. The vertical distribution of fine root biomass decreased with increasing depth except for the top 10–20 cm, where values were lower than immediately below. Production and mortality values were similar between 10 and 50 cm depth. In the 0–10 cm and the 50–60 cm depth intervals, both production and mortality were lower.     

Macrophytic, epipelic and epilithic primary production in a semiarid Mediterranean stream

Summary 1. Primary production by Chara vulgaris and by epipelic and epilithic algal assemblages was measured in a semiarid, Mediterranean stream (Chicamo stream, Murcia, Spain) during one annual cycle. 2. The rates of gross primary production (GPP) and community respiration (CR) were determined for each algal assemblage using oxygen change in chambers. The net daily metabolism (NDM) and the GPPd^?1 : CR₂₄ ratio were estimated by patch‐weighting the assemblage‐level metabolism values. 3. Gross primary production and CR showed significant differences between assemblages and dates. The highest rates were measured in summer and spring, while December was the only month when there were no significant differences in either parameters between assemblages. GPP was strongly correlated with respiration, but not with algal biomass. 4. Chara vulgaris showed the highest mean annual metabolic rates (GPP = 2.80 ± 0.83 gC m^?2 h^?1, CR = 0.76 ± 0.29 gC m^?2 h^?1), followed by the epilithic assemblage (GPP = 1.97 ± 0.73 gC m^?2 h^?1, CR = 0.41 ± 0.12 gC m^?2 h^?1) and epipelic algae (GPP = 1.36 ± 0.22 gC m^?2 h^?1, CR = 0.39 ± 0.06 gC m^?2 h^?1). 5. The epipelic assemblage dominated in terms of biomass (82%) and areal cover (88%), compared with the other primary producers. Epipelic algae contributed 84% of gross primary production and 86% of community respiration in the stream. 6. Mean monthly air temperature was the best single predictor of macrophyte respiration and of epipelic GPP and CR. However, ammonium concentration was the best single predictor of C. vulgaris GPP, and suspended solid concentration of epilithon GPP and CR. 7. Around 70% of the variation in both mean GPP and mean CR was explained by the mean monthly air temperature alone. A multiple regression model that included conductivity, PAR and nitrates in addition to mean monthly air temperature, explained 99.99% of the variation in mean CR. 8. Throughout the year, NDM was positive (mean value 7.03 gC m^?2 day^?1), while the GPP : CR₂₄ ratio was higher than 1, confirming the net autotrophy of the system.     

Influence of floristic composition on the net primary production and dry matter turnover in a tropical grassland

Plant biomass, net primary productivity and dry matter turnover were studied in a grassland situated in a tropical monsoonal climate at Kurukshetra, India (29°58′N, 76°51′E). Based on differences in vegetation in response to microrelief, three stands were distinguished on the study site. The stand I was dominated by Sesbania bispinosa, stand II represented mixed grasses and stand III was dominated by Desmostachya bipinnata. Floristic composition of the three stands revealed the greatest number of species on stand II (75). The study of life form classes indicated a thero-cryptophytic flora. The biomass of live shoots in all the three stands attained a maximum value in September (424–1921 g m^-2) and below ground plant biomass in November (749–1868 g m^-2). The annual above ground net primary production was greatest on stand I (2143 g m^-2) and lowest on stand II (617 g m^-2). The rate of production was highest during the rainy season (15.34 to 3.18 g m^-2 day^-2). Below ground net production ranged from 1592 to 785 g m^-2 y^-2 and the rates were high in winter and summer seasons. Total annual net primary production was estimated to be 3141, 1403, 2493 and 2134 g m^-2 on stands I, II, III and on the grassland as a whole, respectively. The turnover of total plant biomass plus below ground biomass indicated almost a complete replacement of phytomass within the year. The system transfer functions showed greater transfer of material from total net primary production to the shoot compartment during rainy season and to the root compartment during winter and summer seasons.     

Bacterioplanktonic biomass and production in the river Meuse (Belgium)

This paper presents results of bacterial biomass determination by epifluorescence microscopy after acridine orange staining and ³H-thymidine incorporation measurements in the river Meuse. Bacterial production is calculated from thymidine incorporation using an experimental conversion factor (0.5 10¹⁸ bacterial cells produced per mole of thymidine incorporated into macromolecules). Seasonal variations of bacterial biomass and production at two stations are presented. Biomass ranges between 0.05 mgC · 1⁻¹ (in winter) and 0.8 mgC · 1⁻¹ (in summer). The variations of bacterial production seem to be closely linked to those of primary production; values lower than 1 μgC · 1⁻¹ · h⁻¹ are found in winter and high values (> 5 μgC 1⁻¹ · h ⁻¹) in summer. Longitudinal profiles in the Belgian course of the river show important increase of biomass and production from upstream to downstream. Bacterial growth yield (Y) has been determined (Y = 0.3) in order to calculate bacterial carbon uptake from bacterial production.     

Phytoplankton and sea ice algal biomass and physiology during the transition between winter and spring (McMurdo Sound, Antarctica)

The phytoplankton and sea ice algal communities at the end of winter in McMurdo Sound were dominated by Fragilariopsis sublineata, with Thalassiosira antarctica, Melosira adele, Pinnularia quadreata, Entomoneis kjellmannii and heterotrophic dinoflagellates also present. Sea ice algal biomass at the end of winter was very low, only 0.050 ± 0.019 mg chla m⁻² in 2007 and 0.234 ± 0.036 mg chla m⁻² in 2008, but this increased to 0.377 ± 0.078 mg chla m⁻² by early October in 2007 and to 1.07 ± 0.192 by late September in 2008. Under ice phytoplankton biomass remained consistently below 0.1 μg chla l⁻¹ throughout the measuring period in both years. The photosynthetic parameters Fv/Fm, rETRmax and α document microalgal communities that are mostly healthy and well adapted to their low light under ice environment. Our results also suggest that species such as Fragilariopsis sublineata are well adapted to deal with low winter light levels but are unlikely to survive an increase in irradiance, whereas other taxa, such as Thalassiosira antarctica, will do better in a higher light environment.     

Primary production of plankton and decomposition of organic matter in saline lakes of the crimea peninsula

The lakes under study can be classified as eutrophic-hypertrophic waterbodies according to the level of primary production. High concentrations of suspended matter (up to 7326 g/m³) and total phosphorus (up to 5625 mg/m³) were registered in the lakes. The values of the primary production of plankton and the destruction of organic matter do not depend on the absolute concentration of salts in water. Unlike the destruction of organic matter, the primary production of plankton is higher in winter than in summer. Thus, the accumulation of organic matter in saline lakes occurs mainly in winter. The negative relationship between the primary production of plankton and the biomass of the filter feeder Artemia sp. has been determined.     

Effects of dilution stress on the functioning of a saline Mediterranean stream

The effects of seasonality and dilution stress on the functioning of Rambla Salada, a hypersaline Mediterranean stream in SE Spain, were evaluated. The stream is subject to diffuse freshwater inputs from the drainage of intensively irrigated agriculture in the catchment and periodic losses of water through an irrigation channel. Metabolic rates and the biomass of primary producers and consumers were estimated over a 2-year period. During the first year several dilution events occurred, while during the second year the salinity recovery reached predisturbance levels. Functional indicators were compared in the disturbance and recovery salinity periods. Primary production and respiration rates in the Rambla Salada ranged between 0.07–21.05 and 0.19–17.39 g O₂ m⁻² day⁻¹, respectively. The mean values for these variables were 7.35 and 5.48 g O₂ m⁻² day⁻¹, respectively. Mean net daily metabolism rate was 1.87 ± 0.52 g O₂ m⁻² day⁻¹ and mean production/respiration ratio was 2.48 ± 1.1, reflecting autotrophic metabolism. The metabolic rates showed the typical seasonal pattern of Mediterranean open canopy streams. Therefore, gross primary production (GPP) and ecosystem respiration (ER) registered maximum values in summer, intermediate values in spring and autumn and minimum values in winter. The metabolic rates and biomass of consumers were greater in the disturbance period than in the recovery period. However, they did not show significant differences between periods due to their important dependence on seasonal cycle. Seasonality accounted for much of the temporal variability in GPP and ER (76% and 83% in the multiregression models, respectively). Light availability seems to be the most important factor for GPP and ER in the Rambla Salada. Autotrophic biomass responded more to variations in discharge and conductivity than to seasonal variations. In fact, it was severely affected by freshwater inputs after which the epipelic biomass decreased significantly and Cladophora glomerata proliferated rapidly. Epipelic algal biomass was the most sensitive parameter to dilution disturbance. Handling editor: Luigi Naselli-Flores     

The relationship between invertebrate assemblages and available food at forest and pasture sites in three southeastern Australian streams

1. Levels of ash-free dry matter (AFDM) and chlorophyll a in epilithon, benthic participate organic matter (BPOM), invertebrate assemblage composition, and biomass of functional feeding groups were compared in winter and summer at forest and pasture sites in three Victorian streams. 2. Chlorophyll a concentrations of epilithon were significantly higher at pasture than forest sites in winter but not in summer while BPOM was not significantly greater at forest sites in either season. Epilithic biomass as AFDM did not show consistent differences between land uses or seasons. 3. Total biomass of invertebrates did not differ between forest and pasture sites but the biomass of shredders was significantly higher, and that of grazers significantly lower, at forest than pasture sites. A site shaded with an artificial canopy behaved as a forest site for grazers but as a pasture site for shredders. 4. Cluster analyses of invertebrate assemblages grouped pasture sites with forest sites on the same stream at the same season, indicating that assemblage composition was less influenced by land-use differences than by between-stream and seasonal differences. 5. Biomass of functional feeding groups appeared to be a more sensitive indicator of invertebrate assemblage response to land-use alteration than either species diversity/ richness measures, or multivariate assemblage composition measures.     

Light climate and energy flow in the seagrass canopy of Amphibolis griffithii (J.M. Black) den Hartog

 Absorption of light and radiation use efficiency (RUE) were measured in a dense stand of the seagrass Amphibolis griffithii in Warnbro Sound, a temperate marine embayment in southern Western Australia. Total light intercepted by the canopy was measured and compared with dry weight leaf production, under both summer and winter conditions. RUE was found to be higher in winter (1.56 g MJ^–1) than summer (1.01 g MJ^–1). These values are very similar to values measured for annual crop plants and emphasise the value of applying theory developed for terrestrial crop plants to seagrasses. Canopy extinction coefficients were 0.93 m^–1 in winter and 0.44 m^–1 in summer. There were large differences in hours above saturating irradiance (H _sat) between the top (H_sat = 5 h 14 min) and base (18 min) of the canopy in winter. Energy flows in A. griffithii suggest that this species is highly susceptible to short-term perturbations in incident irradience during the winter period as the energy stored within the rhizomes is small relative to daily respiratory demands. Received: 5 October 1995 / Accepted: 14 August 1996     

Relationship between bacteria, phytoplankton and heterotrophic nanoflagellates along the trophic gradient

Bacterial and heterotrophic nanoflagellates (HNF) abundance, as well as bacterial production and chlorophylla levels, were measured at five sites extending from the coastal zone toward the open Adriatic in the period from March to October 1995. The investigated areas were grouped into trophic categories according to concentrations of chlorophylla. All the biotic-para-meters increased along the trophic gradient, leading to eutrophy, but they did not increase at the same rate. The bacterial biomass: phytoplankton biomass (BB: chla) ratio decreased from about 10 in the very oligotrophic area to 0.8 at the eutrophic site. In contrast, the bacterial abundance: HNF abundance ratio (B: HNF) increased from 1000 bacteria per 1 flagellate in the oligotrophic system to 1700 bacteria flagellate⁴ in the eutrophic area. Decreasing BB: chla and increasing B: HNF ratios along the trophic gradient might reflect the different structures of the microbial food web. Relationships between bacterial abundance and production, and chla and HNF showed that bacterial abundance along the trophic gradient was regulated by the interplay between nutrient supply and grazing pressure. But in the oligotrophic system, bacterial abundance was more closely related to bacterial production and chla than in the eutrophic system, suggesting stronger control of bacterial abundance by substrate supply. On the other hand, the coupling between bacteria and HNF, and uncoupling between bacterial abundance and production in the eutrophic system, showed that the importance of bacteriovory increased in richer systems.     

Seasonal variation in light utilisation,biomass production and nutrient removal by wastewater microalgae in a full-scale high-rate algal pond

There has been renewed interest in the combined use of high-rate algal ponds (HRAP) for wastewater treatment and biofuel production. Successful wastewater treatment requires year-round efficient nutrient removal while high microalgal biomass yields are required to make biofuel production cost-effective. This paper investigates the year-round performance of microalgae in a 5-ha demonstration HRAP system treating primary settled wastewater in Christchurch, New Zealand. Microalgal performance was measured in terms of biomass production, nutrient removal efficiency, light absorption and photosynthetic potential on seasonal timescales. Retention time-corrected microalgal biomass (chlorophyll a) varied seasonally, being lowest in autumn and winter (287 and 364 mg m^?3day^?1, respectively) and highest in summer (703 mg m^?3day^?1), while the conversion efficiency of light to biomass was greatest in winter (0.39 mg Chl- a per μmol) and lowest in early summer (0.08 mg Chl- a per μmol). The percentage of ammonium (NH₄–N) removed was highest in spring (79 %) and summer (77 %) and lowest in autumn (47 %) and winter (53 %), while the efficiency of NH₄–N removal per unit biomass was highest in autumn and summer and lowest in winter and spring. Chlorophyll-specific light absorption per unit biomass decreased as total chlorophyll increased, partially due to the package effect, particularly in summer. The proportional increase in the maximum electron transport rate from winter to summer was significantly lower than the proportional increase in the mean light intensity of the water column. We concluded that microalgal growth and nutrient assimilation was constrained in spring and summer and carbon limitation may be the likely cause.     

Assessing Biomass and Production of Bacteria in Eutrophic Lake Mendota, Wisconsin

Estimates were made of the biomass and production of heterotrophic bacteria in the epilimnion of Lake Mendota, Wis. Cell counts were done with epifluorescence microscopy and varied from 3 × 10⁵ bacteria per ml in winter to 3 × 10⁶ bacteria per ml in summer. Cell volumes were measured in scanning electron micrographs. The average cell volume was 0.159 μm³. Annual variations and depth distribution were studied. Production was estimated from the frequency of dividing cells and from dark radioactive sulfate uptake. Annual productivity and daily average productivity were very close with both methods: 107 to 205 g of C per m² per year for sulfate and 89 to 117 g of C per m² per year for frequency of dividing cells. Zooplankton feeding removed 2 to 10% of the bacterial net production annually. When compared with biomass changes and losses due to zooplankton feeding, production values were very high. Therefore, it was suggested that other loss factors have to be more important than zooplankton feeding in controlling the bacterial population. Bacterial heterotrophic production was about 50% of gross primary production.     

Extreme seasonality of litter breakdown in an arctic spring‐fed stream is driven by shredder phenology,not temperature

1. Using degree‐days to calculate ‘temperature‐corrected’ breakdown rates is a useful approach for comparing litter breakdown across sites with different thermal regimes. We used an alternative approach to investigate the importance of temperature by quantifying seasonal patterns in litter breakdown in an arctic spring‐fed stream (Ivishak Spring, North Slope, Alaska) that experiences extreme seasonality in light availability and energy inputs while fluctuations in water temperature are relatively small. 2. We incubated mesh bags of senesced Salix alaxensis litter in Ivishak Spring for successive c. 30‐day periods for 2 years. During our study, water temperature was very stable [5.7 ± 0.03 °C (daily mean ± 1 SE), range 3.7–7.8 °C]. Discharge was only slightly more variable (mean 112 ± 1 L s^?1, range 66–206 L s^?1), with lowest values occurring in late winter. Dissolved nutrient concentrations were low (52–133 μg L^?1, <1–3 μg L^?1, <1–6 μg L^?1 soluble reactive phosphorus) and also showed evidence of seasonality (i.e. highest values in winter). 3. Litter breakdown rates were sharply seasonal, ranging from <0.01 day^?1 in mid‐summer to >0.05 day^?1 in mid‐winter. Invertebrate assemblage structure in litter bags showed pronounced seasonal cyclicity; total invertebrate biomass peaked in summer. Biomass of two dominant shredders (the nemourid stonefly Zapada haysi and the limnephilid caddisfly Ecclisomyia conspersa) showed the opposite trend, however, with mid‐winter peaks in both population biomass and cohort growth rates that closely matched those we observed in litter mass loss. 4. Water temperature appeared to have negligible influence on litter breakdown rates in our study. Seasonal shifts in nutrient uptake may have increased rates of microbial activity in winter. The processing of litter inputs in Ivishak Spring, however, appeared to be most tightly coupled to shredder phenology. Our results demonstrate that extreme seasonality in the processing of allochthonous detritus can occur even in the absence of substantial temperature variation, driven by the activity of shredder taxa that have evolved to take advantage of pulsed organic matter inputs.     

Annual Changes of Abundance and Biomass of Planktonic Ciliates in the Gdańsk Basin,Southern Baltic

Seasonal changes in the species composition, abundance and biomass of planktonic ciliates were determined every 2–3 weeks at two sites of 30 m depth and one location of 105 m depth in the southwestern Gdańsk Basin between January 1987 and January 1988. A total of 40 ciliate taxa were observed during this period. Autotrophic Mesodinium rubrum dominated ciliate abundance and biomass: maximal values of 50 · 10⁻¹ ind. ^1-1 and 65 μg C 1⁻¹ were recorded. The annual mean biomass of M. rubrum comprised 6 to 9% of the annual mean phytoplankton biomass. The highest abundances and biomasses of heterotrophic ciliates were noted at all stations in the spring and summer in the euphotic zone with maximum values of 28 · 10³ ind. 1⁻¹ and 23 μg C 1⁻¹. Three ciliates assemblages were distinguished in the epipelagic layer: large and medium-size non-predatory ciliates, achieving peak abundance in spring and autumn; small-size microphagous ciliates and epibiotic ciliates which were abundant in summer, and large-size predacious ciliates dominating in spring. Below 60 m, a separate deep-water ciliate community composed of Prorodon-like ciliates and Metacystis spp. was found. The ciliate biomass in the 60–105 m layer was similar to the ciliate biomass in the euphotic zone. The heterotrophic ciliate community contributed 10 to 13% to the annual mean zooplankton biomass. The potential annual production of M. rubrum comprised 6 to 9% of the total primary production. Carbon demand of non-predatory ciliates, calculated on the basis of their potential production, was estimated to be equivalent to 12–15% of the gross primary production.