Seasonality,population dynamics and production of planktonic rotifers in Lough Neagh,Northern Ireland

The abundances, population dynamics and production of the rotifer community of Lough Neagh were examined for a three year period. Keratella cochlearis was the most abundant species accounting for over 40% of biomass followed by Polyarthra dolichoptera and Notholca acuminata. The mean standing crop for the rotiferan zooplankton increased in successive years (41, 51, 75 mg dwt m^–2) as did production (1037, 1322, 1417 mg dwt m^–2 y^–1). The seasonal pattern of biomass expression and production varies markedly in different years. Instantaneous birth rates tend to be lower but more consistent for the more abundant species, instantaneous death rates show periods of negative mortality indicating an inadequacy of the model employed but explicable as hatching of resting eggs. K. cochlearis as the most successful species is explained as its perennial appearance and adaptation to the low annual temperature cycle found in the lough. The population succession and the annual occurrence of species differs in each year.     

Changes in phytoplankton physiology and morphology in response to dissolved nutrients in Lough Neagh, N. Ireland

SUMMARY. Cell phosphorus and anthrone-reactive carbohydrate contents were measured over a 6-month period in the phytoplankton of Lough Neagh, N. Ireland. When all the soluble reactive phosphorus had disappeared from the water column, cell phosphorus contents began to decrease, reaching a minimum value of 0.4% ash-free dry weight. At this time, cell carbohydrate contents greatly increased and phosphorus sorption assays showed rapid uptake of orthophosphate. Following a presumed sediment phosphorus release, cell carbohydrate and phosphorus uptake rate decreased greatly and there was a subsequent increase in cell phosphorus content to 1.6% ash-free dry weight. As the cell phosphorus content declined, blue-green algal filaments became progressively shorter. On the basis of these responses it is likely that phosphorus limitation occurred over a 4-week period.     

The rates of sediment-water exchange of oxygen and sediment bioturbation in Lough Neagh,Northern Ireland

Oxygen is transported 30 mm into the sediment at an 8 m depth site in eutrophic Lough Neagh by the irrigational activities of the benthic fauna. Faunal activity also mixes the upper 20 mm of sediment. Sediment oxygen uptake rate, redox potential-depth profile and the chlorophylla concentration were measured in the upper sediment layers from February to November 1979. Chlorophylla input to the sediment, following the Spring phytoplankton maximum, remained in the 0–1 cm sediment layer but did cause the redox potential profile to change from one with potentials around 400 mV in the upper 50 mm to one with a strong gradient over the 0–30 mm region. The start of benthic faunal activity in May caused the chlorophylla to be mixed into the 1–2 cm layer and also caused oxygen to be transported into the sediment at a rate sufficient to change the redox potential back to its initial state. The biodiffusion coefficient for solids in the upper 20 mm was estimated to be 6 × 10⁻⁸ cm² s⁻¹. Oxygen transport in the pore, waters of the upper sediment layers was considered to be best described as advection, caused by the irrigational activities of the benthic fauna.     

Interactions between the roach,Rutilus rutilus,and waterfowl populations of Lough Neagh,Northern Ireland

Synopsis Following the introduction of roach, Rutilus rutilus, to a large eutrophic lake in ca. 1973, a subsequent increase in the abundance of this cyprinid through the 1970s was accompanied by a decline in the numbers of one of the lake&s most abundant overwintering waterfowl, the tufted duck, Aythya fuligula, and an increase in overwintering piscivorous great crested grebes, Podiceps cristatus. We suggest that these contrasting trends are causally related and that competition for benthos and increased prey availability are the mechanisms responsible for the changes in the tufted duck and grebe populations respectively. In agreement with these hypotheses, a reduction in the roach population during the mid 1980s was accompanied by a recovery of tufted ducks and a decline of grebes.     

The food of the pollan, Coregonus autumnalis pollan Thompson, in Lough Neagh, Northern Ireland

Stomach contents of pollan caught monthly throughout the year were examined. Stomach fullness was significantly correlated with water temperature. Adult pollan fed on bottom fauna, mainly chironomid larvae, in October-March, chironomid pupae in April and on Daphnia spp. in May-September. Immature (0+ and 1 +) fish also ate other cladocerans and copepods. While there is no evidence for selection of any benthic prey species, adult pollan appear to be highly selective feeders on Daphnia spp. in summer.     

Seasonal variation in growth, mortality and fat stores of roach and perch in Lough Neagh, Northern Ireland

The annual cycle for roach ( Rutilus rutilus ) and perch ( Perca fluviatilis ) in Lough Neagh, Northern Ireland can be divided into warm 'growing' and cold 'non-growing' seasons. Summer water temperatures declined between 1991–1993, as did the sizes of 0+ fish of both species. Small overwinter increases in length occurred after poor summers due, in roach, to size-selective mortality. Fat content was size-dependent and increased over the summer in both species. All sizes of roach but only small perch lost fat overwinter. Dead roach collected towards the end of the 1992/3 winter had significantly less fat than live individuals. The allocation of energy to growth (and ultimately reproduction) and to fat (and overwinter survival) was modelled as a tradeoff, assuming that fat has twice the energy density of protein and using observed summer and winter changes in body composition. The predicted allocation optima were close to observed values but, for roach, this entailed a substantial survivorship cost.     

Evidence for the co-existence of separate strains or species of Ligula in Lough Neagh,Northern Ireland

LIGULA (Cestoda: Pseudophyllidea) infections in gudgeon (Gobio gobio) and roach (Rutilus rutilus) differ markedly in the pathology that is observed in the host, particularly with respect to a tissue response and the extent of inhibition of gonadal development. The entire internal transcribed spacer (ITS) region (ITS-1, 5.8S and ITS-2) and the large subunit domains D1-D3 were sequenced and compared in parasites from these fish from Lough Neagh, Northern Ireland, together with a single specimen from minnow (Phoxinus phoxinus) from Wales. Sufficient differences were observed between parasites from R. rutilus and G. gobio to support the suggestion that they may represent different strains/species. In contrast, Ligula from P. phoxinus closely resembled those from R. rutilus. Ligula infections in G. gobio were recorded prior to the introduction of R. rutilus. The co-existence of separate strains or species of Ligula in Lough Neagh probably resulted from the introduction of R. rutilus to these waters, correlated with an increase in the number of great crested grebes (Podiceps cristatus).     

Age determination and growth of the pollan, Coregonus autumnalis pollan Thompson, of Lough Neagh, Northern Ireland

Scales from Lough Neagh pollan display a large number of checks, making age determination difficult. Sclerite counts showed that an annual check is formed on scales in May and a second accessory check in most young fish in October. The method of ageing from scales was supported by inspection of length-frequency plots and by following the growth of pollan in their first 2 years of life. The body-scale relationship was curvilinear. Back-calculation showed that pollan of both sexes attain a fork length of 29 cm in 5 years (1 ∞=28.9 cm; k = 0.65; l ₀= -0.06 year). There is no evidence that annual growth rates have changed since 1965. Possible environmental causes of scale check formation are discussed.     

The seasonal cycle of condition in the pollan, Coregonus autumnalis pollan Thompson, of Lough Neagh, Northern Ireland

The length–weight relationship for Lough Neagh pollan was log W= 3·2 log L –2·24. Covariance analysis indicated that there were no differences between the sexes, with maturity, or between years. While a seasonal cycle in relative condition was in phase with gonad development during late summer and autumn, changes in somatic condition presented a more complex pattern. Somatic condition increased in spring, as did the quantity and diversity of food consumed. Despite good conditions for feeding and growth, there was an autumn fall in somatic condition, caused by the diversion of energy to gonad growth.     

The Spawning and Early Life History of the Pollan (Coregonus pollan THOMPSON) in Lough Neagh,Northern Ireland

Studies had been carried out on the local non-migratory whitefish. Spawning behaviour and light conditions on the spawning ground are recorded. After artificial spawning, embryonal development took place under controlled conditions and the larvae were reared. Detailed characteristics of swim bladder creation and differentiation of the digestive tract are given. Distribution and number of pollan larvae near the spawning area and in the open lake were examined. Data are presented concerning the growth rates of larvae and fry in captivity and under lake conditions. Ecological data related to other coregonids are discussed and compared with the present study.     

The interaction of components controlling net phytoplankton photosynthesis in a well-mixed lake (Lough Neagh, Northern Ireland)

The homogeneous distribution of the phytoplankton in a shallow (mean depth 8·6 m) unstratified lake, L. Neagh, Northern Ireland, facilitated the study of the interaction of components controlling gross photosynthesis per unit area. These included the photosynthetic capacity, the phytoplankton content of the euphotic zone, and a logarithmic function describing the effective radiation input. These factors were analysed for two sites, the open lake and Kinnego Bay, which respectively had standing crops of up to 90 and 300 mg chlorophyll a m^?3 and maximum daily rates of gross integral photosynthesis of 11·7 and 15·6 g O₂ m^?2 day^?1. Values are reduced by the high contribution to light attenuation by non-algal sources, which increases at low standing crops particularly in winter, when values of integral photosynthesis decrease to 0·5 g O₂ m^?2 day^?1. This relative change is the result of self-shading behaviour of the phytoplankton altering the crop content of the euphotic zone at different population densities. Changes in the irradiance function, incorporating day length, are largely responsible for the changes in daily rates of integral gross photosynthesis; as daily irradiance is also a determinant of water temperature, it exerts further influence through the photosynthetic capacity which was strongly correlated with temperature. Much of the gain in gross photosynthesis resulting from higher photosynthetic capacity may not be reflected in a higher net column photosynthesis, because of the greater proportional rise in respiration with temperature. The balance in the water column between respiration losses and photosynthetic input may frequently alter since the ratio of illuminated to dark zones is between 1/4 to 1/5 in the open lake, and small shifts in any of the controlling features may result in conditions unfavourable for growth. This is analysed especially for the increase of diatoms in spring, when small modifications of the underwater light field can delay growth.     

Relationship between elver recruitment and changes in the sex ratio of silver eels Anguilla anguilla L. migrating from Lough Neagh, Northern Ireland

Between 1932–1947 and from 1960 onwards, elvers have been trapped near the mouth of the River Bann, Northern Ireland, and released into Lough Neagh. Each period of elver transport has been followed by a marked increase in the proportion of male silver eels migrating from the lough. Catches of silver eels were sampled on several nights each year from 1965–1974, and the lengths of a total of 20358 eels measured showed a progressive increase in the percentage of male eels from 9.3-86.0 % during this period. Various reasons for this change were examined. The different ages at which male and female eels migrate to the sea was not important. There was no evidence to support the hypothesis that male elvers normally remain in estuarine conditions, and their transport to the lough was therefore unnatural. An increasing fishing effort for yellow eels, such as occurred following the introduction of trawling in 1960, would favour males since small eels were returned to the lough. It was not thought, however, that this was a major cause of the change in sex ratio. Instead, elver transport appeared to be directly implicated, possibly by the overstocking of Lough Neagh, and the phenotypic determination of progressively more male eels, but the evidence for this suggestion was inconclusive.     

Biological and ecological effects of a Ligula intestinalis (L.) infestation of the gudgeon, Gobiogobio (L.), in Lough Neagh, Northern Ireland

A study was made of the effects of a Ligula intestinalis infestation on the biology and distribution of gudgeon in Lough Neagh, Northern Ireland. During the summer months, when the majority of the gudgeon population was restricted to the inshore areas of the lough, Ligula was present in all age (size) classes of the host although levels of infestation were highest in the oldest individuals. While there was no significant effect on the length-weight relationship of the gudgeon, the presence of the parasite did adversely affect gonad size in both sexes. Ligula also influenced the distribution of the gudgeon population by decreasing the swimming depth of infested individuals and by delaying their autumnal migration to the deeper offshore parts of the lough.     

Vegetation, landscape and human activity in Midland Ireland: mire and lake records from the Lough Kinale-Derragh Lough area, Central Ireland

A high-resolution pollen record for the Holocene has been obtained from Derragh Bog, a small raised mire located on a peninsula in Lough Kinale-Derragh Lough, in Central Ireland as part of the Discovery Programme (Ireland) Lake Settlements Project. The data are compared with two lower resolution diagrams, one obtained from Derragh Lough and one from adjacent to a crannog in Lough Kinale. The general trends of vegetation change are similar and indicate that landscape-scale clearance did not occur until the Medieval period (ca. a.d. 800–900). There are, however, significant differences between the diagrams due primarily to core location and taphonomy, including pollen source area. Only the pollen profile from Derragh Bog reveals an unusually well represented multi-phase primary decline in Ulmus ca. 3500–3100 b.c. (4800–4750¹⁴C b.p.) which is associated with the first arable farming in the area. The pollen diagram indicates a rapid, and almost complete, clearance of a stand of Ulmus with some Quercus on the Derragh peninsula, arable cultivation in the clearing and then abandonment by mobile/shifting late Neolithic farmers. Subsequently there are a number of clearance phases which allow the colonisation of the area by Fraxinus and are probably associated with pastoral activity. The pollen sequence from adjacent to a crannog in Lough Kinale shows clear evidence of the construction and use of the crannog for the storage of crops (Hordeum and Avena) whereas the Derragh Bog diagram and the diagram from Derragh Lough reflect the growth of the mire. This study reveals that in this landscape the record from a small mire shows changes in prehistoric vegetation caused by human agriculture that are not detectable in the lake sequences. Although in part this is due to the higher temporal resolution and more consistent and complete chronology for the mire, the most important factor is the closer proximity of the raised mire sequence to the dry land. However, the pollen sequence from adjacent to a crannog does provide detailed evidence of the construction and function of the site. It is concluded that in order to ascertain a complete picture of vegetation changes in a lowland shallow lake-dominated landscape, cores from both the lake and surrounding small mires should be analysed.     

Silica and diatom growth in Lough Neagh: the importance of internal recycling

1. Lough Neagh is a large eutrophic lake with a high dissolved silica (SiO2) concentration arising from the basaltic rock in parts of the catchment and the normal winter maximum concentration is over 8 mg L−1. Based on frequent observations between 1974 and 1997, the annual cycles of SiO2 input, uptake and release are explored.
2. Large spring blooms of the planktonic diatoms Aulacoseira subarctica Haworth and Stephanodiscus astraea occur every year and are terminated by SiO2 depletion. Although there are periods when one diatom species has dominated the other, over the period as a whole, the two diatoms appear to be in a stable coexistence.
3. A model of the effect of river inputs on lake concentration shows that without SiO2 release from the sediment, the maximum spring dissolved SiO2 concentration (and hence diatom crop) would be only about one-third of that actually observed.
4. It is concluded that within-lake processes play a large role and are potentially more variable than catchment processes in determining the available SiO2 in Lough Neagh.
5. The role of benthic animals, especially Chironomus anthracinus, in the sediment SiO2 release process is discussed. Field data suggest the SiO2 release rate is highly sensitive to temperature, but this could be partly caused by an interaction between temperature and animal activity.     

Aspects of nitrogen fixation in Lough Neagh. II. Competition between Aphanizomenon flos-aquae, Oscillatoria redekei and Oscillatoria agardhii

SUMMARY.

• 1 The effect of nutrient availability on the growth of natural samples of Lough Neagh plankton (Aphanizomenon flos-aquae, initial innoculum 0.98 mm³ l^-1, Oscillatoria redekei, 2.52 mm³ l^-1 and Oscillatoria agardhii, 4.96 mm³ l^-1) was studied, using ecologically realistic conditions of temperature (15°C), nitrate-N (<0.35g m^-3) and orthophosphate-P (<0.55g m^-3), in which the combinations –N–P, –N+P, +N–P, and +N+P were used.
• 2 After 8 days A. flos-aquae had become dominant in treatments lacking N. After 12 days with N available the mean yields of O. redekei (37.7 mm³ l^-1) and O. agardhii (10.82 mm³ l^-1) were significantly greater (P=5%) than their respective yields (3.6 and 2.6 mm³ l^-1) in the absence of N. Somewhat surprisingly the mean yield of A. flos-aquae when N was present (0.15 mm³ l^-1), was significantly less (P=5%) than in the absence of N (13.26 mm³l^-1).
• 3 The relative rates and duration of growth, the availability of P and N, and heterocyst frequency as an index of nitrogen fixation are considered in relation to the seasonal succession and dominance of these three principal cyanophyte members of the Lough Neagh phytoplankton.

     

Aspects of nitrogen fixation in Lough Neagh. I. Acetylene reduction and the frequency of Aphanizomenon flos-aquae heterocysts

SUMMARY.

• 1 An investigation was made of the relationships between rates of nitrogen fixation (acetylene reduction), Aphanizomenon flosaquae heterocysts, NO₃-N concentration and other environmental factors which occurred in Lough Neagh between 1969 and 1972.
• 2 The maxima of cells differentiated as heterocysts (approximately 5% (1970), and approximately 7% (1971 and 1972) occurred when NO₃-N concentration was very low or undetectable.
• 3 Acetylene reduction was detectable between 16 May and 8 September 1972 but was not measured in the other years. Rates were greatest in the euphotic zone and it appeared that the nitrogenase activity was closely linked to a light dependent system.
• 4 Areal rates of acetylene reduction were highly correlated with Aphanizomenon heterocyst concentration (r=0.92). There was some indication that a release of phosphorus from the sediments might have stimulated Aphanizomenon nitrogen fixation on one occasion at the end of a calm period. Results show that the most likely conditions for the development of large crops of heterocystous species in Lough Neagh include a scarcity of NO₃-N and a plentiful supply of phosphorus.

     

Cormorant (Phalacrocorax carbo [L]) populations and patterns of abundance at breeding and feeding sites in Northern Ireland,with particular reference to Lough Neagh

Breeding cormorants, Phalacrocorax carbo, in Northern Ireland remain seasonally dependent on a coastal environment where they can be censused with accuracy, but numbers in other habitats and at other times of the year are less certain. This study establishes the long term and regional patterns of abundance at breeding and feeding localities, which might in turn be related to diet.Birds were regularly observed and counted at a variety of feeding sites, and some aspects of their breeding success and fledgling diet evaluated at the largest N.Ireland breeding colony.Numbers of breeding birds increased dramatically over a period of eight years but recently show signs of declining. There is likely to be a dynamic relationship between populations of a tapeworm (Ligula intestinalis L.), the numbers of roach (Rutilus rutilus [L.]) and cormorant populations feeding at Lough Neagh, the largest lake in the British Isles. While roach are not a major component in the diet of fledglings, the large numbers of cormorants feeding at the perimeter of L. Neagh suggest that changes in roach populations will affect birds most acutely during or following the winter. We suggest that this might result in a reduction in the proportion of the population volunteering to breed in the subsequent season, but require further data. The longer term effects of one fish species on cormorant populations are unlikely to be critical, since these birds are highly opportunist.In other habitats cormorant numbers are either very stable (estuary) or variable (river) depending on the seasonal and annual availability of their prey. There is no evidence for a systematic seasonal shift in habitat, as suggested by other studies.