The effect of salmon farming on the benthos of a Scottish sea loch

The effects of waste from a salmon farm on the benthos of a fjordic sea loch on the western coast of Scotland have been studied. Within 3 m of the floating cages the sediment was highly reducing, and dissolved oxygen content of the water overlying the sediment ranged from 35 to 75% saturation. At ⩾ 15 m from the cages the sediment was oxygenated, and the dissolved oxygen content of the water overlying the sediment was 50–85% saturation. Sedimentary redox potential and dissolved oxygen content of bottom water showed a seasonal variation. The benthic fauna showed marked changes in species number, species diversity, faunal abundance, and biomass in the region of the fish farm, with four zones of effect identified. Directly beneath, and up to the edge of the cages, there was an azoic zone. A highly enriched zone, dominated byCapitella capitata (Fabricius) andScolelepis fuliginosa (Claparède), occurred from the edge of the cages out to as 8 m. A slightly enriched ″transitional ″ zone occurred at ⩽ 25 m, and a “clean” zone at distances > 25 m. This study showed that salmon farming had similar effects on the benthos as other forms of organic enrichment, but the effects were limited to a small area in the immediate vicinity of the cages     

Compensation effect of phytoplankton on nutrients from a sewage outfall in summer

This paper investigates the relationship between nutrients, chlorophyll a and physical variables in the upper Saronikos Gulf, an oligotrophic marine environment south of the Greater Athens Metropolitan Area. Phosphate, silicate, ammonia, nitrite, nitrate, temperature, salinity and dissolved oxygen were determined at eight stations on 9 occasions during summer 1982. A thermocline led to the stratification of the water column and the pycnocline was related to the thermocline. The values of oxygen were more or less normal. The Eutrophication of the seawater in the vicinity of the sewage outfall was demonstrated by surface levels of chlorophyll a being forty to two hundred times above background. This parameter provides evidence for a high phytoplankton standing stock. However, there were no appreciable differences between the nutrients in the outfall area and those in the background. This suggests rapid uptake of nutrients and/or effective dispersal from the outfall.     

Impact of intensive cage fish farming on the phytoplankton and periphyton of a Scottish freshwater loch

Nutrients, phytoplankton and periphyton were monitored in a 71 ha shallow, unstratified lake used for intensive cage culture of rainbow trout. Inorganic nitrogen, ortho-phosphate and suspended solids were significantly higher near the cages and the bottom and, although declining during summer, nutrients did not reach levels which limit phytoplankton growth. Microcystis aeruginosa dominated the phytoplankton, with surface chlorophyll a reaching 189 µg l^–1 in August, but with no subsequent bloom collapse or deoxygenation. A sub-dominant community of vernal diatoms and Pediastrum spp. persisted. Periphyton was dominated by Melosira italica-subarctica. Algal species and water quality showed the lake to be highly eutrophic. Chlorophyll values predicted from a phosphorus-dependent eutrophication model agreed with observations but light limitation by self-shading and suspended farm wastes, aided by wind-induced turbulence, is believed to control algal growth rates and biomass. Implications for environmental management of intensive freshwater cage farms are discussed.     

The contribution of epipelon to total sediment microalgae in a shallow temperate eutrophic loch (Loch Leven, Scotland)

Benthic microalgae are known to perform important ecosystem functions in shallow lakes. As such it is important to understand the environmental variables responsible for regulating community structure, positioning and biomass. We tested the hypothesis that the positioning (across a depth gradient of 2–22 m overlying water depth) and relative biomass (determined using bulk and lens tissue harvested chlorophyll (Chl) a concentrations) of the epipelon community would vary independently with season (12 monthly samples) and across natural gradients of light and habitat disturbance relative to the total benthic algal community (i.e. all viable microalgae in the surface sediments) in a shallow eutrophic loch. Total sediment microalgal Chl a concentrations (TS-Chl; range: 5–874 μg Chl a g⁻¹ dw) were highest in winter and in the deepest site (20 m overlying water depth), apparently as a result of phytoplanktonic settling and sediment focussing processes. Epipelic Chl a concentrations (Epi-Chl; range: <0.10–6.0 μg Chl a g⁻¹ dw) were highest in winter/spring, a period when water clarity was highest and TS-Chl lowest. Principal components analysis highlighted strong associations between Epi-Chl and sites of intermediate depths (2.5–5.5 m) in all seasons except autumn/winter. Autumn/winter represented the season with the highest average wind speeds preceding sampling, during which the highest Epi-Chl concentrations were associated with the deepest sites. Epi-Chl was associated with intermediate light and habitat disturbance during spring/summer and summer/autumn and varied positively with habitat disturbance, only, in autumn/winter and winter/spring. The epipelon community structure also varied with depth; diatoms dominated shallow water sediments, cyanobacteria dominated deep water sediments, and sediments at sites of intermediate depth returned the highest biovolume estimates and the most diverse communities. This study has strengthened the hypothesis that the structure and biomass of benthic microalgal communities in lakes are regulated by habitat disturbance and water clarity, both of which are expected to respond to climate change and eutrophication. The degree to which these structural responses reflect functional performance requires clarification.     

The feeding relationships of the larger fish species in a Scottish sea loch

Patterns within a fish assemblage have been examined from catches obtained over a period of 17 months using fixed nets and rod and line sampling. A total of 19 species were caught with summer samples dominated by dab and saithe and winter samples by whiting. Cod were caught regularly throughout the year. Catches at the study site of dab and whiting occurred mainly at night but for cod and saithe catches occurred throughout the day. There was no indication ofa crepuscular or tidal rhythm for any species. Differences between our own observations and those from other published studies lead us to conclude that no species-specific generalizations about feeding periodicities can be made. Feeding patterns are more likely to be determined by site-specific features of particular habitats and their associated prey and predator assemblages.
Examination of stomach contents data using a variety of methods indicates that conclusions are highly dependent on the form of the resource matrix used. However, our analyses indicate that during summer cod and dab feed on a much narrower range of prey than saithe and that there is significant overlap in their diets if a resource matrix based on percentage occurrence of prey species is used. Nevertheless, most diet overlap values for the species examined were low (between 0.1 and 0.2) suggesting that competition for food is unlikely. To date, no experiments have been performed to test the hypothesis that cod and dab compete for food.     

Effects of fertilization and irrigation on productivity, plant nutrient contents and soil nutrients in southern Mongolia

The study attempts to evaluate the effect of fertilization and irrigation on steppe productivity in dry southern Mongolian desert-steppes. We conducted an irrigation- and NPK fertilization experiment, irrigating at levels of +100 mm and fertilizers at amounts equivalent to 20 gN (m² year)^?1 in a factorial design. We tested the effects on soil nutrients and biomass production. Nutrients in plant tissue were analysed for Stipa krylovii and S. gobica, and for mixed sub-samples of total above- and belowground biomass. Available P and K and total K increased in the soil after fertilization while irrigation reduced total N. Biomass yield almost tripled and inflorescence numbers increased by factors of 4?C8 due to fertilization while irrigation alone had very restricted effects and only increased biomass of Agropyron cristatum. Nutrient content of biomass was elevated on fertilized plots. Results indicate that steppe productivity is severely restricted by nutrient availability even under ambient precipitation levels, raising the question whether nutrient withdrawal caused by current land use practices has detrimental effects on pasture productivity. The anticipated beneficial effect of increasing water availability however could not be confirmed. Whether there is an improvement in productivity due to increasing rainfall, as predicted by some climate change models, will depend on the distribution and intensity of rain events.     

Experimental study of the effect of nutrients on phytoplankton of the shallow highly eutrophic Lake Nero

Experimental studies of the effect of phosphorus, nitrogen, and silicon on the phytoplankton of the shallow highly eutrophic Lake Nero have been carried out. It is shown that the enrichment of lake water with nitrogen leads to an increased concentration of chlorophyll a and the growth of the biomass of dominant algae species. Phosphorus limits the abundance of a certain species of algae; the limiting role of silicon in the development of spring phytoplankton has not been experimentaly confirmed.     

The light-climate of Loch Leven, a shallow Scottish lake, in relation to primary production by phytoplankton

Seasonal changes in incident irradiance and underwater light penetration at Loch Leven from 1968 to 1971 are discussed in relation to the photosynthetic behaviour and crop density of phytoplankton. Light extinction was highest in the blue and lowest in the orange spectral regions, a pattern typical of other turbid waters. Euphotic depth varied between 1·2 and 7·4 m and was on average c. three times the Secchi disc transparency. Underwater light extinction depended chiefly on phytoplankton crop density (estimated as chlorophyll a). Despite the shallowness and wind-exposed situation of the loch there was no evidence of appreciable light extinction due to sediment disturbance. Possible causes of variability in the relationship between the minimum vertical extinction coefficient (k _min) and the concentration of chlorophyll a are discussed. The value of k_s, the increment in k_min per unit increment in algal concentration, was estimated from field data as 0·0086 In units per mg chl a/m² and from laboratory spectroradiometer data as 0·0079 In units per mg chl a/m². These k_s values imply theoretical upper limits for the amount of chlorophyll a in the euphotic zone (Σn _max) of 430 and 468 mg chl a/m², respectively. Observed euphotic chlorophyll a contents (Σn) were sometimes close to these upper limits. Typical photosynthesis/depth profiles are described. Profile area is shown to be related to the logarithm of the ratio between surface-penetrating irradiance (I_o') and the irradiance (I_k) defining the onset of light-saturation of photosynthesis. Standardized profiles, plotted on a common scale of ‘optical depth’, are used to illustrate the relatively minor influence of variations in I_o' and I_k on hourly rates of photosynthesis per unit area. The saturation parameter (I_k) generally increased as photosynthetic capacity (P_max) increased; the temperature-dependence of I_k is explained by the temperature-dependence of the enzyme-controlled (dark) reactions of photosynthesis, which control P_max. A spring peak in the ratio between surface penetrating irradiance (I_o') and I_k is interpreted as a result of a lag in the seasonal increase in water temperature with increase in surface irradiance. The gradient (K') of the linear light-limited region of the photosynthesis-irradiance curve showed little variation and had an average value of 0·31 mg O₂/mg chl a.h per 1 W/m² (PAR). Interactions between mixed depth, underwater light extinction and phytoplankton productivity are discussed; comparisons are made with other shallow, optically deep lakes.     

The effect of cytochalasin B and D on the fertilization of sea urchins

Two electrical events occur across the plasma membrane of the sea urchin egg during the interaction with the fertilizing spermatozoon; the first, a step-like depolarization pecedes the fertilization potential (FP) by 11 sec at 20°C. Cytochalasins B and D alter these events in several ways depending upon dose and time of exposure of the gametes. When both the egg and the spermatozoon are exposed to these microfilament-active drugs, the former for minutes whilst the latter in the order of seconds, the delay between the two electrical events is increased twofold. We suggest that a microfilament-involved process is occurring during this delay period which, although itself not altering the electrical properties of the plasma membrane, determines the time of initiation of the FP. Pretreating spermatozoa with higher doses of either CB or CD, inhibits fertilization completely or lowers the successful collision rate (α); we suggest that the two drugs lower α by affecting the acrosomal region of the spermatozoon. Although both drugs increase the delay period to the same degree CD has fewer secondary effects than CB.     

Temperature modulated effects of nutrients on phytoplankton changes in a mountain lake

Piburger See, a dimictic mountain lake in Austria, experienced moderate cultural eutrophication in the 1950s. Lake restoration led to a re-oligotrophication in the 1990s with a decrease in seasonal phytoplankton biovolume until the late 1990s, but a reversed trend from the early 2000s onwards. We hypothesize that recent changes in phytoplankton biomass and functional structure are triggered by changes in lake nitrogen and silica concentrations, and we expect climate-related factors to modulate the trophic status of Piburger See. Phytoplankton data were analyzed by non-metric multidimensional scaling (NMDS) applied on biovolume of morpho-functional groups, combined with correlation analyses of environmental variables. Since the 2000s, short-term changes in phytoplankton of Piburger See were explained by varying concentrations and ratios of nitrogen and silica, while the inter-annual variability in phytoplankton species composition was rather attributed to superimposed rising water temperature and lake thermal stability. Our results underline the co-dominant role of phosphorus and nitrogen as phytoplankton drivers in lakes that experience periods of nitrogen limitation. The combined impact of nutrients and climate on phytoplankton development can thus mimic short-term increases in the trophic level of less productive lakes.     

Distribution of phytoplankton and nutrients in relation to different water masses in the area around Elephant Island, Antarctica

During January March 1996 the U.S. Antarctic Marine Living Resources program carried out an extensive multidisciplinary study in a 40,000 km² sampling grid around Elephant Island, Antarctica. The physical, chemical, optical, and biological characteristics of the upper water column (0–750 m) were determined at 91 hydrographic stations. Analysis of the temperature and salinity data showed that six different hydrographic zones could be differentiated. The biological (phytoplankton distribution and abundance) and chemical (inorganic nutrient concentrations) data also showed characteristic differences within each of these six zones. In spite of high concentrations of inorganic N, P, and Si in all six zones, all stations in the northwest portion of the sampling grid (Drake Passage waters) showed very low chlorophyll-a concentrations in surface waters and a sub-surface maximum at increased depth. As stations in this zone have a relatively stable upper mixed layer of 40 m, excess macro-nutrients, and adequate solar radiation for maximal photosynthetic rates, this suggests that rates of primary production in this zone are limited by a micro-nutrient such as Fe. Phytoplankton abundance was much greater in the Bransfield Strait, in waters influenced by Bellingshausen Sea Water, and in the frontal zones where these water masses mix with Drake Passage waters. Relatively low and deeply distributed phytoplankton abundance was found at all stations in the southeastern portion of our sampling grid, where the upper water column was very weakly stratified and showed the characteristics of Weddell Sea water. The areas of enhanced phytoplankton biomass in the AMLR sampling grid roughly correspond to the areas where krill are generally also found in greater abundance. The overall biological productivity of the Elephant Island region would thus appear to be dependent upon the circulation patterns of the major water masses that intrude into this area. Received: 28 October 1996 / Accepted: 20 January 1997     

Studies of 13C in the foliage reveal interactions between nutrients and water in forest fertilization experiments

The severity of a root rot disease of cereals, caused by Rhizoctonia solani Kühn AG8, was inversely correlated to the Zn status of plants in field studies in 1989 and 1990. In 1989, a preliminary survey was conducted in a farmer's field in South Australia where Zn deficiency and disease were both widespread. Zn concentration in Spear wheat plants at the 3-leaf to early tillering stage was negatively correlated with severity of the disease. For the elevent elements analysed, a correlation matrix showed that Zn had the highest, and only significant (R²=0.52^**) association with disease. The effect of Zn applications and their residual value on disease severity was further studied in a long-term field experiment in 1989 and 1990 to which Zn had been applied in 1986. There was a decrease in the area of Rhizoctonia bare patch as Zn rate was increased, a result consistent with the field survey results; the recommended rate of 2.5 kg Zn ha^–1 reduced the area affected by bare patch from 42% to 21% of the total crop area compared with no Zn application, overcame Zn deficiency and increased grain yield from 1.1 to 2.8 t ha^–1. In 1990, fresh Zn application treatments were applied to trial plots designed for this purpose, in order to compare the response with the older Zn treatments applied in 1986. The areas of bare patch in the older Zn treatments were approximately 5% greater than those in the fresh Zn treatments. The results are consistent with the hypothesis that Zn deficient plants are more susceptible to root rot caused by R. solani. Testing this hypothesis is the subject of a companion paper.     

Modern pollen from the catchment and surficial sediments of a Scottish sea loch (fjord)

Modern pollen was extracted both from moss polster samples collected from a range of sites across the land surface of the Loch Sunart catchment, north‐west Scotland and from a number of marine sediment‐water interface sites in the sea loch. Comparisons between the pollen results and the major vegetation types growing in the catchment area revealed that in general the moss polsters contained a localised picture of the vegetation whilst the sea loch sediments varied much less from sample to sample and better represented a regional picture of the vegetation. It was anticipated that the pollen in the sea loch samples would be in a much poorer state of preservation because of the many pathways through which it travels in order to become incorporated into the sediments of the loch. However, this proved not to be the case and the study demonstrates that marine sediments of the kind found in Loch Sunart have the potential to provide a new source of Holocene vegetation data which is as good as the freshwater lake sediments that have been preferentially sampled in the past.     

Reduced Lepeophtheirus salmonis larval abundance in a sea loch on the west coast of Scotland between 2002 and 2006

A survey for planktonic sea louse larvae was carried out in Loch Shieldaig, Scotland, between 2002 and 2006, and spanned 2 successive production cycles (Cycles 1 and 2) at a local Atlantic salmon Salmo salar L. farm. The vast majority of the caligid copepodids recovered were Lepeophtheirus salmonis; however, the methodology was unable to determine the species of the caligid nauplii. Greatest densities of nauplii were found at the sampling station adjacent to the salmon farm, and larval densities were low during the fallow period of both cycles. Peaks in nauplius densities occurred around the same time in the 2 cycles, but the peaks were significantly lower during Cycle 2 than Cycle 1. Lepeophtheirus salmonis copepodid densities varied temporally, but not spatially. During most of Cycle 2, copepodid densities were significantly lower than those recovered during Cycle 1. Numbers of gravid L. salmonis at the local salmon farm correlated significantly with densities of louse nauplii and L. salmonis copepodids in the water at time lags of 0 and 1 wk, and 1 and 2 wk, respectively. This survey demonstrated a reduction in densities of L. salmonis larvae in the plankton (an indication of L. salmonis infectious pressure) between the 2 cycles and indicated that the farm was an important source of L. salmonis larvae. The application of anti-louse treatments using emamectin benzoate reduced the numbers of gravid L. salmonis at the farm, and this was the main factor influencing the apparent reduction in L. salmonis infectious pressure in the loch between cycles.     

Small-scale vertical distribution of phytoplankton, nutrients and sulphide below the oxycline of a mesotrophic lake

A characteristic vertical sequence of phytoplankton populationswas observed below the metalimnetic oxycline of a stratified,mesotrophic lake. Ceratium spp., Closterium acutum and Aphanizomenonflos- aquae were present in the epilimnion but had distinctpopulation maxima in the microaerobic chemocline. Below thesepopulations, Cryptomonas phaseolus, Planktothrix clathrata,Pseudanabaena catenata and Limnothrix sp. followed each otherin the transition zone between the chemocline and the sulphide-containinghypolimnion. The dominating populations of P. clathrata andP. catenata caused a deep chlorophyll maximum. Phytoplanktonstructure was determined by the vertical gradients of sulphideand light. Compared with the epilimnion, nutrient availabilitywas not fundamentally better below the oxycline but the algaemight have benefited from reduced grazing pressure in theirhabitat.