Distribution, production and role of aquatic macrophytes in a southern Michigan marl lake

A typical marl lake of the Upper Great Lakes region has very few quantitatively important aquatic macrophytes. The macrophytes, however, dominate the total primary production of the lake. Submersed vegetation is extremely sparse on the shallow (less than I m) marl bench that characterizes the littoral of these lakes, and is completely dominated by one. little-known species (Scirpus subterminalis Torr.) between 1 and 7 m. A detailed investigation of the spatial and seasonal distribution of macrophytic species and biomass showed that S. subterminalis strongly dominated the lake (79% of total biomass). S. suhterminalis represented an almost pure stand (to 200 g m^?2 mean annual ash-free dry weight) at all times of the year at intermediate depths of macrophytic growth (1–6 m). Two species of Chara (of eight varieties and forms) were present in significant quantities (12% of total biomass; to 100 g m^?2) but were severely limited to shallow depths (0-S-l m) and protected areas. Several annual submersed angiosperms were present (9% of total biomass), but only two species were quantitatively important. Potamogeton illinoensis Morong. and P. praelongus Wulfen formed brief summer peaks (less than 100 g m^?2) at 3 and 4–6 m, respectively. A striking feature of the seasonal biomass distribution of Scirpus subterminalis was the higher, viable biomass (to 150g m^?2) throughout the winter under ice cover. Cyclic fluctuations of the S. subterminalis populations were discerned at different depths, each with different periodicities. The population at 2 m exhibited a fall peak; that at 4 m had a summer maximum. The lowest overall biomass of S. subterminalis occurred in the 2 m population in June. Chara populations at 0–2 m also exhibited a relatively constant biomass throughout the year. The appearance of Nitella at 7 m in July-October and of Chara at 5 m in September-October was interpreted as an interaction between light, thermal, and carbon stratification. Estimates of macrophytic productivity of perennial (‘evergreen’) species populations whose biomass remains relatively constant throughout the year were made employing several different methods of calculation and turnover factors. All methods resulted in productivity estimates in good agreement with the conservative value of 178 g m^?2 year^?1 for the entire lake. In comparison to the other components (phyto-planktonic, epiphytic and epipelic algae) of the primary production of Lawrence Lake, the aquatic macrophytes constituted a major portion (anuual mean 82·77 g C m^?2 year^?1 or 48·3 %) of the total production of the lake. The low diversity but relatively high quantitative importance of macrophytes in marl lakes is attributed to an adverse dissolved inorganic and organic chemical milieu which inhibits phytoplanktonic production and allows only certain adapted macrophytes to develop strongly. The phenomenon of perennial biomass levels throughout the year is believed to be much more common than previously suspected and has iikely resulted from adaptations of submersed macrophytes to ameliorated conditions of water and temperatures relative to the terrestrial situation in winter.     

Life cycle and production of Chironomidae (Diptera) from Lake Banyoles (NE Spain)

1. The life cycles and annual production of the eight most abundant species of chironomids (Prodadius cf. choreus, Tanypus punctipennis, Chironomus bernensis, Chironomus gr. plumosus, Cladopelma virescens, Microchironomus tencr, Tanytarsus gr. lestagei, and Cladotanytarsus atridorsum) were studied from sublittoral and profundal samples taken monthly in Lake Banyoles during 1987 at five sampling stations (depths ranging from 5 to 20 m). 2. The number of generations per year deduced from instar-frequency data varied from one to four, depending on the species, lake basin and depth. Annual temperature range, dissolved oxygen in the stratified period and presence of sulphide are the key factors that may explain the differences in the number of generations. 3. Production estimates were calculated using the size-frequency (SF) method corrected for the number of generations (SFG), and the increment-summation method (IS) when cohorts could be clearly deduced. 4. Production calculated with the SFG method gave results which were comparable with those of the IS method using smoothed-survivorship curves in the three species for which the use of the IS method was possible (C. virescens, M. tener and C. atridorsum). Using these methods production was estimated to range from 23–70 mg AFDW (ash-free dry weight) m^?2 yr^?1 at 12 m to 74–275 mg AFDW m^?2 yr^?1 in the sublittoral zone of the lake (5-m depth). 5. Calculation of production for the other five species using the size-frequency method with the number of generations (SFG) deduced from monthly instar-frequency data gave values ranging from 12 mg AFDW m^?2 yr^?1 (Chironomus bernensis at 20 m depth) to 3.75 g AFDW m^?2 yr^?1(Prodadius cf. choreus at 12 m). 6. Total chironomid production (with the SFG method) varied from 0.8 to 5 g AFDW m^?2 yr^?1 in the profundal and sublittoral, respectively. At each sampling station two species groups accounted for most of the production: Prodadius cf. choreus and Chironomus spp. Annual production/biomass ratio (P/B) varied from very high values for Prodadius (between 11 and 27, as four generations completed each year) to very low values for Chironomus gr. plumosus (2.20), which completed only one generation each year. The annual production of P. cf. choreus in Lake Banyoles is higher than any reported in the literature due to the completion of four generations and to the high densities of this species.     

Density of red alder (Alnus rubra) in headwaters influences invertebrate and detritus subsidies to downstream fish habitats in Alaska

We investigated the influence of red alder (Alnus rubra) stand density in upland, riparian forests on invertebrate and detritus transport from fishless headwater streams to downstream, salmonid habitats in southeastern Alaska. Red alder commonly regenerates after soil disturbance (such as from natural landsliding or timber harvesting), and is common along streams in varying densities, but its effect on food delivery from headwater channels to downstream salmonid habitats is not clear. Fluvial transport of invertebrates and detritus was measured at 13 sites in spring, summer and fall during two years (2000–2001). The 13 streams encompassed a riparian red alder density gradient (1–82% canopy cover or 0–53% basal area) growing amongst young-growth conifer (45-yr-old stands that regenerated after forest clearcutting). Sites with more riparian red alder exported significantly more invertebrates than did sites with little alder (mean range across 1–82% alder gradient was about 1–4 invertebrates m^?3 water, and 0.1–1 mg invertebrates m^?3 water, respectively). Three-quarters of the invertebrates were of aquatic origin; the remainder was of terrestrial origin. Aquatic taxa were positively related to the alder density gradient, while terrestrially-derived taxa were not. Streams with more riparian alder also exported significantly more detritus than streams with less alder (mean range across 1–82% alder gradient was 0.01–0.06 g detritus m^?3 water). Based on these data, we predict that headwater streams with more riparian alder will provide more invertebrates and support more downstream fish biomass than those basins with little or no riparian alder, provided these downstream food webs fully utilize this resource subsidy.     

Survival and growth of salmon, Salmo salar (L.), planted in a Scottish stream

Salmon eggs and unfed fry were planted in reaches (total length 2.8 km, mean width 4 m) of a Scottish stream between 1971 and 1977 and their subsequent progress was studied by sampling 16 sections (areas 38–126 m²) of the stream. Brown trout are the only fish which spawn in the stream, waterfalls and a dam near its mouth preventing adult salmon and sea-trout passing upstream. There were no restraints on the downstream movement of fish except in 1977, when a fry trap was operated. In 1971 and 1974 boxes each containing 300 eggs were buried in groups of 3–6. In other years fry were evenly distributed at 3.6–29.3 m^?2. At the end of the first growing season, salmon occurred at decreasing population densities for a distance of 600 m below the plantings, but after two growing seasons there was little remaining indication of their pattern of dispersion when planted. Rates of survival between planting and the end of the growing season were 9.4–31%. Survival when eggs were planted (11.1–14.8%) was not affected by the numbers planted together at one point (900–1800) or the distance apart of groups of boxes (10–85 m). When fry were planted the instantaneous mortality rate (M) of the 0₊ salmon during their first growing season was related to the initial stocking density (D_p) by the formula M= 0.00637 + 0.00444 log₁₀D_p. Twenty-two to 88% of 0₊ salmon present at the end of the growing season were still surviving in the stream as 1₊ fish one year later. In 1973–1976 only a small number of 2₊ salmon occurred, the majority having migrated between the end of the second growing season and the following spring. There were more 2₊ salmon in 1977 and 1978 resulting from higher stocking densities in 1975 and 1976 and slower growth. Trout of several age classes were present but their population densities were never high (<0.6 m^?2). Salmon reached a greater size than trout by the end of the first growing season. Their mean weight (W^o, g) at this time was inversely related to their population density (D_o No. m^?2) and the biomass (B₁, g m^?2) of 1₊ salmon present, giving the relationship log₁₀w^o= 0.6584–0.0558 D₀-0.0352B₁. The mean weight of 1₊ salmon tended to be highest in sections where the 0₊ salmon had reached a relatively large size the previous year. When a reach of the stream was planted twice (11 and 30 May 1977) with salmon fry (total 13.9 m^?2) at the same stage of development, M during the first growing season was 0.0099 per day. This was less than that of fry in a control (M= 0.0107) where the stocking density was lower (6.8 m^?2) and also less than in previous years when single planting rates of approximately 14 m^?2 were used (M=0.0115). The double planting resulted in a wide range of lengths of 0₊ salmon in September and the highest biomass values encountered during all experiments.     

Saltcedar (Tamarix ramosissima) invasion alters organic matter dynamics in a desert stream

1. We investigated the impacts of saltcedar invasion on organic matter dynamics in a spring‐fed stream (Jackrabbit Spring) in the Mojave Desert of southern Nevada, U.S.A., by experimentally manipulating saltcedar abundance. 2. Saltcedar heavily shaded Jackrabbit Spring and shifted the dominant organic matter inputs from autochthonous production that was available throughout the year to allochthonous saltcedar leaf litter that was strongly pulsed in the autumn. Specifically, reaches dominated by saltcedar had allochthonous litter inputs of 299 g ash free dry mass (AFDM) m^?2 year^?1, macrophyte production of 15 g AFDM m^?2 year^?1 and algal production of 400 g AFDM m^?2 year^?1, while reaches dominated by native riparian vegetation or where saltcedar had been experimentally removed had allochthonous litter inputs of 7–34 g AFDM m^?2 year^?1, macrophyte production of 118–425 g AFDM m^?2 year^?1 and algal production of 640–900 g AFDM m^?2 year^?1. 3. A leaf litter breakdown study indicated that saltcedar also altered decomposition in Jackrabbit Spring, mainly through its influence on litter quality rather than by altering the environment for decomposition. Decomposition rates for saltcedar were lower than for ash (Fraxinus velutina), the dominant native allochthonous litter type, but faster than for bulrush (Scirpus americanus), the dominant macrophyte in this system.     

Earthworm Succession in Afforested Colliery Spoil Heaps in the Sokolov Region, Czech Republic

Earthworm communities were studied at six heap sites representing a chronosequence of Alnus glutinosa (black alder) stands (age 3–62 years) and compared with those on an unameliorated heap and in an alder stand (60 years old) on natural soil. Spoil heaps in the open‐cast coal mining area near Sokolov (northwestern Bohemia) were mainly reclaimed using afforestation. No earthworms were found on the virgin heap. Young plots were colonized by euryecious epigeic earthworms (i.e., those living above soil surface), but higher proportions of endogeic species (i.e., soil dwellers), did not appear until after more than 30 years of succession. The density and biomass of earthworms increased from the youngest stand (67 individuals/m²; 5 g/m²) to the older ones (e.g., 407 ind/m²; 13 g/m² in the 23‐year‐old stand). However, both parameters were low in the oldest stand (35 ind/m²; 3 g/m²), but this may have been the result of extensive soil disturbance. Earthworm populations were often higher in reclaimed sites than in the control alder stand (150 ind/m²; 7 g/m²). However, the community structures were different, with the control being dominated by the litter‐feeding species, Dendrobaena vejdovskyi.     

COMPARISONS OF SALT-MARSH FUCOID PRODUCTION ESTIMATED FROM THREE DIFFERENT INDICES1

Production of Ascophyllum nodosum (L.) LeJolis ecads and Fuscus vesiculosus L. was calculated from measurements of in situ growth, seasonal variations in standing-crops and seasonal variations in photosynthetic capacity. A computer model for predicting daily, monthly and yearly net production from photosynthesis data was constructed. This model used daily irradiation, actual biomass of algae/m² contributing to production and photosynthesis vs. light intensity relationships as data inputs. Comparison of production estimated from in situ growth, standing-crops and photosynthesis indicated that both marsh fucoids turn over biomass twice per year. Total net production of both fucoids, estimated from photosynthesis data, was ca. 315 g C · m^?2· yr^?1. On the other hand, production of both fucoids calculated from standing-crop data was only 155 g C · m^?2· yr^?1.     

Light use efficiency of dry matter gain in five macro-lichens: relative impact of microclimate conditions and species-specific traits

Relations between irradiance (I) and lichen growth were investigated for five macro‐lichens growing at two sites in Sweden. The lichens represented different mycobiont–photobiont associations, two morphologies (foliose, fruticose) and two life forms (epiphytic, terricolous). The lichens were transplanted at two geographically distant sites in Sweden (1000 km apart) from Sept 1995 to Sept 1996 in their typical microhabitats, where microclimate and growth were followed. Between April/May and Sept 96, the terricolous species had a dry matter gain of 0·2 to 0·4 g (g DW)^–1 and the epiphytes 0·01 to 0·02 g (g DW)^–1. When related to area, growth amounted to 30 to 70 g m^?2 for the terricolous species and to 1 to 4 g m^?2 for the epiphytes. There was a strong correlation between growth and intercepted irradiance when the lichens were wet (I_wet), with 0·2 to 1·1 g lichen dry matter being produced per MJ solar energy. Across the 10 sets of transplants, light use efficiencies of dry matter yield (e) ranged between 0·5 and 2%, using an energy equivalent of 17·5 kJ g^?1 of lichen dry matter. The higher productivity of the terricolous species was due to longer periods with thallus water contents sufficient for metabolic activity and because of the higher mean photon flux densities of their microhabitat. A four‐fold difference in photosynthetic capacity among the species was also important. It is concluded that lichen dry matter gain was primarily related to net carbon gain during metabolically active periods, which was determined by light duration, photon flux density and photosynthetic capacity.     

In situ litter decomposition and litter quality in a Mojave Desert ecosystem: effects of elevated atmospheric CO2 and interannual climate variability

Rising atmospheric CO₂ has been predicted to reduce litter decomposition as a result of CO₂‐induced reductions in litter quality. However, available data have not supported this hypothesis in mesic ecosystems, and no data are available for desert or semi‐arid ecosystems, which account for more than 35% of the Earth's land area. The objective of our study was to explore controls on litter decomposition in the Mojave Desert using elevated CO₂ and interannual climate variability as driving environmental factors. In particular, we sought to evaluate the extent to which decomposition is modulated by litter chemistry (C:N) and litter species and tissue composition. Naturally senesced litter was collected from each of nine 25 m diameter experimental plots, with six plots exposed to ambient [CO₂] or 367 μL CO₂ L^?1 and three plots continuously fumigated with elevated [CO₂] (550 μL CO₂ L^?1) using FACE technology beginning in April 1997. All litter collected in 1998 (a wet, or El Niño year; 306 mm precipitation) was pooled as was litter collected in 1999 (a dry year; 94 mm). Samples were allowed to decompose for 4 and 12 months starting in May 2001 in mesh litterbags in the locations from which litter was collected. Decomposition of litter produced under elevated CO₂ and ambient CO₂ did not differ. Litter produced in the wetter year showed more rapid initial decomposition (over the first 4 months) than that produced in the drier year (27±2% yr^?1 or 7.8±0.7 g m^?2 yr^?1 for 1998 litter; 18±3% yr^?1 or 2.2±0.4 g m^?2 yr^?1 for 1999 litter). C:N ratios of litter produced under elevated CO₂ (wet year: 37±0.5; dry year: 42±2.5) were higher than those of litter produced under ambient CO₂ (wet year: 34±1.1; dry year: 35±1.4). Litter production in the wet year (amb. CO₂: 25.1±1.1 g m^?2 yr^?1; elev. CO₂: 35.0±1.1 g m^?2 yr^?1) was more than twice as high as that in the dry year (amb. CO₂: 11.6±1.7 g m^?2, elev. CO₂: 13.3±3.4 g m^?2), and contained a greater proportion of Lycium pallidum and a lower proportion of Larrea tridentata than litter produced in the dry year. Decomposition, viewed across all treatments, decreased with increasing C:N ratios, decreased with increasing proportions of Larrea tridentata and increased with increasing proportions of Lycium pallidum and Lycium andersonii. Because litter C:N did not vary by litter production year, and CO₂ did not alter decomposition or litter species/tissue composition, it is likely that the impact of year‐to‐year variation in precipitation on the proportion of key plant species in the litter may be the most important way in which litter decomposition will be modulated in the Mojave Desert under future rising atmospheric CO₂.     

A quantitative diving survey of benthic vegetation and fauna in Lake Kariba, a tropical man-made lake

SUMMARY. 1. The biomass distribution of submerged vascular vegetation and benthic fauna were investigated by diving in Lake Kariba. The vegetation was well correlated with transparency of the water. Maximum biomass (1400 g dryweight m^?2) and a depth penetration of 6 m were found in areas little influenced by river inflow, while these were only 110 g m^?2 and 2 m, respectively, in the basin receiving water from the Zambezi river. 2. The lake is mesotrophic-oligotrophic. The total biomass for the lake was 101,000 tons dryweight of rooted vegetation composed of Lagarosiphon ilicifolius Oberm. (52%), Najas pectinata (Parl) Magnus (33%), Vatlisneria aethiopica Frenzl (11%), Ceratophyllum demersum L. (3%) and Potamogeton octandrus L. (0.5%). Average plant biomass for the potentially colonizable depth zone of 0-12 m and for the total lake amounted to 79.9 g m^?2 and 18.8 g m^?2, respectively. 3. The distribution of the benthic fauna generally followed that of the vegetation. The total animal biomass of 118,840 tons dryweight. including shells, consisted of mussels (95.8%), snails (4.1%) and insect larvae (0.1%). Four species of mussels were found: Caelatura mossambicensis (von Martens) (81% of mussel biomass), Corbicula africana (Krauss) (9%). Mutela dubia (Gmelin) (5%) and Aspatharia wahlbergii (Krauss) (5%). Among the snails Melnoides tuberculuta (Müller), Cleopatra spp. and Bellamya capillata (Frauenfeld) dominated. 4. The average animal biomass was high compared to most other lakes perhaps due to lack of predators. For the colonizable 0-12 m depth interval and the total lake it was 96.2 g m^?2 including shells (15.0 g m^?2 shell-free dryweight) and 22.6 g m^?2 including shells (3.4 g m^?2 shell-free dryweight). respectively. Biomass of plants and animals was even higher prior to the recent lowering of the water level by 7 m, which was estimated to have stranded 84,000 tons of mussels on the shore.     

Photosynthetic responses to photosynthetically active radiation and temperature including chilling‐light stress on the heteromorphic life history stages of a brown alga,Cladosiphon okamuranus (Chordariaceae) from Ryukyu Islands,Japan

Photosynthetic responses to temperature and photosynthetically active radiation (PAR) were investigated on the heteromorphic life history stages (macroscopic and microscopic stages) of an edible Japanese brown alga, Cladosiphon okamuranus from the Ryukyu Islands. Measurements were carried out by using optical dissolved oxygen sensors and a pulse‐amplitude modulated fluorometer. Maximum net photosynthetic rates and other parameters of the Photosynthesis – PAR curves at 28°C were somewhat similar in both life history stages, without characteristic photoinhibition at 1000 μmol photons m^?2 s^?1. Results of oxygenic gross photosynthesis and dark respiration experiments over a temperature range of 8–40°C revealed similar temperature optima for both stages (29.7°C, macroscopic stage; 30.3°C, microscopic stage), which support their observed occurrences in the habitat during summer. Maximum quantum yields of photosystem II (PSII ) (F _v /F _m ) were relatively stable at low temperatures with the highest at 15.1°C for the macroscopic stage and at 16.5°C for the microscopic stage; but dropped at higher temperatures especially above 28°C. Continuous exposures (6 h) to 200 and 1000 μmol photons m^?2 s^?1 at 8, 16, and 28°C revealed greater depressions in effective quantum yields of PSII (Φ _PSII ) of the microscopic stage at 8°C, as well as its F _v /F _m that barely increased after 6 h of dark acclimation. Whereas post‐dark acclimation F _v /F _m of both stages exposed to low PAR fairly recovered at 28°C, suggesting their photosynthetic tolerance to such high temperature. Under natural conditions, both heteromorphic stages of C. okamuranus may persist throughout the year in this region. Beyond its northern limit of distribution, the microscopic stage of this species may suffer from photodamage, as enhanced by low winter temperatures; hence, its restricted occurrence.     

The dynamics of peat accumulation on bogs: mass balance of hummocks and hollows and its variation throughout a millennium

This study concerns the mass balance in hummocks and hollows on three ombrotrophic boreonemoral bogs in both a short (ca 10 yr) and long (1000 yr) time scale. Nitrogen, ¹⁴C. and ^?210Pb are used to establish detailed time scales and to estimate productivity and decay losses in tour different microtopographical units: hummocks with either Sphagnum or lichens and hollows with either Sphagnum lawns or bare hollows. The accumulation of N and ²¹⁰Pb was greater in hummocks than in hollows. The litter input was higher in Sphagnum hummocks (170-210 g m^?2yr^?1) than in lawns (110-145 g m ^?2- yr^?1) while its decay rate (0.011 -0.014 yr^?1) did not differ. The arotelm was deeper in Sphagnum hummocks than in lawns but because of less compaction in lawns, neither residence time (80 100 yr) nor decay losses (70-75%) differed. Productivity in lichen hummocks and bare hollows was insignificant and the mass balance negative. It is concluded that the higher productivity in Sphugnum hummocks maintains the microtopography on the mire surface. The mass balance in hummocks will determine not only the development in hollows but also the rise of the ground water mound, and the height increment of a bog. The addition of mass to the catotelm has generally been less in hollows than in hummocks. Since 800 BP the overall input to the catotelm has decreased from about 150 to < 50 g m ^?2 yr^?1 due to longer residence time increasing losses through decay in the acrotelm from < 20% to 70% and is the result of either climatic changes or autogenic processes in the bog ecosystem. Before recent centuries the whole bog surface must have been covered with Sphagnum mosses, forming an overall input of litter as large as in the recent Sphagnum hummocks and lawns. Due to the present lesser cover of peat forming mosses (20-50% of the surface), the recent overall input of peat-forming litter is only 50-65 g m^?2 yr^?1. The bogs no longer act as sinks for carbon since the input of carbon only just covers the losses as CH₄ and CO₂.     

Differential responses of production and respiration to temperature and moisture drive the carbon balance across a climatic gradient in New Mexico

Southwestern North America faces an imminent transition to a warmer, more arid climate, and it is critical to understand how these changes will affect the carbon balance of southwest ecosystems. In order to test our hypothesis that differential responses of production and respiration to temperature and moisture shape the carbon balance across a range of spatio‐temporal scales, we quantified net ecosystem exchange (NEE) of CO₂ and carbon storage across the New Mexico Elevational Gradient, which consists of six eddy‐covariance sites representing biomes ranging from desert to subalpine conifer forest. Within sites, hotter and drier conditions were associated with an increasing advantage of respiration relative to production such that daily carbon uptake peaked at intermediate temperatures – with carbon release often occurring on the hottest days – and increased with soil moisture. Across sites, biotic adaptations modified but did not override the dominant effects of climate. Carbon uptake increased with decreasing temperature and increasing precipitation across the elevational gradient; NEE ranged from a source of ～30 g C m^?2 yr^?1 in the desert grassland to a sink of ～350 g C m^?2 yr^?1 in the subalpine conifer forest. Total aboveground carbon storage increased dramatically with elevation, ranging from 186 g C m^?2 in the desert grassland to 26 600 g C m^?2 in the subalpine conifer forest. These results make sense in the context of global patterns in NEE and biomass storage, and support that increasing temperature and decreasing moisture shift the carbon balance of ecosystems in favor of respiration, such that the potential for ecosystems to sequester and store carbon is reduced under hot and/or dry conditions. This implies that projected climate change will trigger a substantial net release of carbon in these New Mexico ecosystems (～3 Gt CO₂ statewide by the end of the century), thereby acting as a positive feedback to climate change.     

The subalpine vegetation of Mt. Vysokaya, central Sikhote-Alin

The subalpine vegetation structure of Mt. Vysokaya, the Central Sikhote-Alin, is described. This vegetation consists mainly of subalpine spruce-fir forest, a complex of subalpine meadows, shrubs, groves of Betula lanata (B. ermanii s.l.), krummholz of Pinus pumila and alpine tundras. Significant disturbances in the vegetation structure were noted, especially in the forest-tundra ecotone accompanying a sharp reduction of the belts of Betula lanata and Pinus pumila. The altitudinal level of the upper timberline reaches 1600 m a.s.l. which is 250 m less than the expected altitude calculated by Kira's warmth index. An undergrowth of scattered trees of Picea and Betula are growing up to the mountain top. Based on these data and a review of the literature, we concluded that a catastrophic lowering of the timberline and devastation of the subalpine vegetation belt occurred several centuries ago, probably as result of fires.     

Factors affecting photosynthesis and its seasonal variation in the seagrasses Cymodocea nodosa (Ucria) Aschers,and Posidonia oceanica (L.) Delile in the mediterranean

Measurements of photosynthesis, dark respiration, and leaf chlorophyll content were made in the laboratory on both shallow (1 to 5 m) and deep (25 to 33 m) leaves of Cymooceu nodosa (Ucria) Aschers, and Posidonia oceanica (L.) Delile in Malta in April and August. Light saturated photosynthetic rates in Cymodocea were similar in spring (18 μg C cm^?2h^?1) and summer (25μg Ccm^?2h^?1) if the 9 C increase in water temperature in summer is taken into account: however, photosynthetic rates in Posidonia were higher in spring than in summer, especially in shallow leaves which fixed ≈ 10 μg C cm^?2h^?1 in spring but less than half that in summer when rates of carbon accretion were close to compensation point. Levels of irradiance at which photosynthesis was light saturated (Iλ were ≈ 3 mW cm^?2 PAR for Cymodocea and 2 mW cm^?2 PAR for Posidonia: underwater irradiance at the lower depth limit for these plants (≈33 m) was ≈3 mW cm^?2 PAR. corresponding closely to the saturation irradiances. Compensation irradiance for both species was between 0.3 and 0.5 mW cm^?2 PAR.Photosynthesis in both species had a temperature optimum at about 30 C (slightly higher in Cymodocea in summer). Dark respiration rates were generally similar in spring and summer, in the region of 3 μg C cm^?2 h^?1 in Cymodocea and 1.5 to 2 μg C cm^?2 h^?1 in Posidonia. Increase in dark respiration rates with increased temperature was considerably greater in spring than in summer in both species. Photosynthesis was directly proportional to chlorophyll content in Posidonia in the range encountered (up to 58 μg Chl cm^?2) and the summer reduction in photosynthesis was closely correlated with reduction in chlorophyll content. It seems unlikely that environmental factors such as seasonal changes in light intensity, nutrient availability or water temperature were directly responsible for this loss of chlorophyll and it is suggested that this is a manifestation of general leaf senescence, probably induced by daylength changes but possibly enhanced by increased water temperature. Cymodocea showed a similar reduction in chlorophyll content in summer but this was not reflected in reduced photosynthesis. Thus, although Cymodocea may grow rapidly throughout the spring and summer with an overall productivity of 3.6 g C m^?2 day^?1 in shallow water, the luxuriant growths of Posidonia must develop in the first half of the year when a dense meadow may produce up to 2.1 g C m^?2 day^?1 in shallow water, declining to ?0.6 g C m^?2 day^?1 in summer.     

Turgor-sensitive transport in developing seeds of legumes: the role of the stage of development and the use of excised vs. attached seed coats

Abstract After removal of the embryo from developing seeds of Vicia faba L. and Pisum sativum L., the ‘empty’ ovules were filled with a substitute medium (pH 5.5) and the effect of the osmolality of this solution on assimilate transport was exandned. In pulse-labelling experiments with a mixture of [³H]sucrose and [¹⁴C]α-andnoisobutyric acid (AIB), a solute concentration of 400 mol m^?3 (100 mol m^3? sucrose + 300 mol m^?3 mannitol) was too low to maintain sugar and andno acid transport into empty ovules of V. faba in a very early stage of development (embryo dry weight < 100 mg) on the same level as transport into intact ovules within the same fruit. A 550-mol m^?3 solution could maintain the normal rate of transport. In experiments with seeds in a more advanced stage of development (embryo dry weight > 250 mg), transport of labelled sucrose and AIB into empty ovules filled with a 400-mol m^?3 solution was practically equal to transport into intact ovules within the same fruit. Experiments without isotopes, on sugar and andno acid release from the seed coat, confirmed the important role of the osmotic environment. A very low osmolality of the solution (e.g. 50 mol m^?3 mannitol) enhanced net efflux of assimilates from excised seed coats and cotyledons, by inhibiting resorption from the apoplast.     

Mediterranean warming triggers seagrass (Posidonia oceanica) shoot mortality

Rapid warming of the Mediterranean Sea threatens marine biodiversity, particularly key ecosystems already stressed by other impacts such as Posidonia oceanica meadows. A 6‐year monitoring of seawater temperature and annual P. oceanica shoot demography at Cabrera Archipelago National Park (Balearic Islands, Western Mediterranean) allowed us to determine if warming influenced shoot mortality and recruitment rates of seagrasses growing in relative pristine environments. The average annual maximum temperature for 2002–2006 was 1 °C above temperatures recorded in 1988–1999 (26.6 °C), two heat waves impacted the region (with seawater warming up to 28.83 °C in 2003 and to 28.54 °C in 2006) and the cumulative temperature anomaly, above the 1988–1999 mean annual maximum temperature, during the growing season (i.e. degree‐days) ranged between 0 °C in 2002 and 70 °C in 2003. Median annual P. oceanica shoot mortality rates varied from 0.067 year^?1 in 2002 to 0.123 year^?1 in 2003, and exceeded recruitment rates in all stations and years except in shallow stations for year 2004. Interannual fluctuations in shoot recruitment were independent of seawater warming (P>0.05). P. oceanica meadows experienced a decline throughout the study period at an average rate of ?0.050±0.020 year^?1. Interannual variability in P. oceanica shoot mortality was coupled (R²>0.40) to seawater warming variability and increasing water depth: shoot mortality rates increased by 0.022 year^?1 (i.e. an additional 2% year^?1) for each additional degree of annual maximum temperature and by 0.001 year^?1 (i.e. 0.1% year^?1) for each accumulated degree water temperature remained above 26.6 °C during the growing season. These results demonstrate that P. oceanica meadows are highly vulnerable to warming, which can induce steep declines in shoot abundance as well indicating that climate change poses a significant threat to this important habitat.     

Effects of benthivorous bream (Abramis brama) and carp (Cyprinus carpio) on sediment resuspension and concentrations of nutrients and chlorophyll a

1. The effect of benthivorous bream and carp on sediment resuspension and the concentrations of nutrients and chlorophyll a were studied in sixteen experimental ponds (mean depth 1m, mean area 0.1 ha, sandy clay/clay sediment), stocked with bream or carp at densities varying from 0 to 500 kg ha^?1. Planktivorous perch (Perca fluviatilis L.) were added to some ponds to suppress zooplankton. 2. Suspended sediment concentrations increased linearly with biomass of benthivorous fish. Bream caused an increase of 46 g sediment m^?2 day^?1 per 100kg bream ha^?1 and a reduction of 0.38m^?1 in reciprocal Secchi disc depth, corresponding to an increase in the extinction coefficient of 0.34m^?1. 3. No relationship was found between size of fish and amount of resuspension, but the effect of bream was twice as great as that of carp. Benthivorous feeding was reduced in May because alternative food (zooplankton) was available. 4. Assuming a linear relationship, chlorophyll a level increased by 9.0 μgI^?1, total P by 0.03mgl^?1 and Kjeldahl-N by 0.48mgl^?1 per 100kg bream ha^?1. Silicate, chlorophyll a, total P and total N were all positively correlated with fish biomass, but orthophosphate showed no correlation.     

Estimates of soil respiration and net primary production of three forests at different succession stages in South China

Soil respiration (heterotropic and autotropic respiration, R_g) and aboveground litter fall carbon were measured at three forests at different succession (early, middle and advanced) stages in Dinghushan Biosphere Reserve, Southern China. It was found that the soil respiration increases exponentially with soil temperature at 5 cm depth (T_s) according to the relation R_g=a exp(bT_s), and the more advanced forest community during succession has a higher value of a because of higher litter carbon input than the forests at early or middle succession stages. It was also found that the monthly soil respiration is linearly correlated with the aboveground litter carbon input of the previous month. Using measurements of aboveground litter and soil respiration, the net primary productions (NPPs) of three forests were estimated using nonlinear inversion. They are 475, 678 and 1148 g C m^?2 yr^?1 for the Masson pine forest (MPF), coniferous and broad‐leaf mixed forest (MF) and subtropical monsoon evergreen broad‐leaf forest (MEBF), respectively, in year 2003/2004, of which 54%, 37% and 62% are belowground NPP for those three respective forests if no change in live plant biomass is assumed. After taking account of the decrease in live plant biomass, we estimated the NPP of the subtropical MEBF is 970 g C m^?2 yr^?1 in year 2003/2004. Total amount of carbon allocated below ground for plant roots is 388 g C m^?2 yr^?1 for the MPF, 504 g C m^?2 yr^?1 for the coniferous and broad‐leaf MF and 1254 g C m^?2 yr^?1 for the subtropical MEBF in 2003/2004. Our results support the hypothesis that the amount of carbon allocation belowground increases during forest succession.     

Modeling to discern nitrogen fertilization impacts on carbon sequestration in a Pacific Northwest Douglas‐fir forest in the first‐postfertilization year

This study investigated how nitrogen (N) fertilization with 200 kg N ha^?1 of urea affected ecosystem carbon (C) sequestration in the first‐postfertilization year in a Pacific Northwest Douglas‐fir (Pseudotsuga menziesii) stand on the basis of multiyear eddy‐covariance (EC) and soil‐chamber measurements before and after fertilization in combination with ecosystem modeling. The approach uses a data‐model fusion technique which encompasses both model parameter optimization and data assimilation and minimizes the effects of interannual climatic perturbations and focuses on the biotic and abiotic factors controlling seasonal C fluxes using a prefertilization 9‐year‐long time series of EC data (1998–2006). A process‐based ecosystem model was optimized using the half‐hourly data measured during 1998–2005, and the optimized model was validated using measurements made in 2006 and further applied to predict C fluxes for 2007 assuming the stand was not fertilized. The N fertilization effects on C sequestration were then obtained as differences between modeled (unfertilized stand) and EC or soil‐chamber measured (fertilized stand) C component fluxes. Results indicate that annual net ecosystem productivity in the first‐post‐N fertilization year increased by～83%, from 302 ± 19 to 552 ± 36 g m^?2 yr^?1, which resulted primarily from an increase in annual gross primary productivity of～8%, from 1938 ± 22 to 2095 ± 29 g m^?2 yr^?1 concurrent with a decrease in annual ecosystem respiration (R_e) of～5.7%, from 1636 ± 17 to 1543 ± 31 g m^?2 yr^?1. Moreover, with respect to respiration, model results showed that the fertilizer‐induced reduction in R_e (～93 g m^?2 yr^?1) principally resulted from the decrease in soil respiration R_s (～62 g m^?2 yr^?1).