Hydrobiologia 175: 195-212, 1989 0 1989 Kluwer Academic Publishers. Printed in Belgium 195 A study of Lough Corrib, western Ireland and its phytoplankton Enda P. Mooney Department of Botany, University College, Galway, Ireland Received 7 October 1987; in revised form 21 March 1988; accepted 30 March 1988 Key words: Lough Corrib, Ireland, phytoplankton succession,nutrient cycling Abstract During 1980/1981,a twelve month study was carried out on the phytoplankton of Lough Corrib, a large freshwater lake in western Ireland. Cell counts indicated low phytoplankton productivity and this was reflectedin low chlorophyll a values and high Secchireadings. Diatoms predominatedin spring and were succeededby blue-greenalgae in late summer. The spring diatom crop was apparentlylimited by either silicate or nitrogen. Mineral uptake by rooting macrophytes and benthic algae and phosphaterecycling as a result of bottom sediment disturbancemay be important elementsin the nutrient cycling regime of the lake. Differencesin morphologybetween the upper and lower basins of the lake are reflected,to some extent, in speciescomposition and areal productivity. Introduction these areas,surrounded by non-calcareous,older and more acid rocks, the greatestdepths occur. Geography and geology Lough Corrib, situated to the north of Galway Catchment and rivers city, western Ireland (Fig. l), is Ireland’s second The L. Corrib catchment (Fig. l), comprising largestfreshwater lake. It is 40 km in axial length, 3320km2, may be divided into three sub-catch- is extremely irregular in breadth and has an area ments. The largest drains into lower L. Corrib, of 183km2, excluding islands. The water level of and includes the catchments of the rivers Clare the lake lies mainly between8.5 and 9.2 m above and Cregg.These drain relatively rich but by and Ordnance Datum (O.D.). A narrowing in the large, unintensively managedlowland farmlands, centre divides the lake into a larger northern and to the east of the lake, which overlie a bedrock of a smaller southern basin. The latter is shallow karstic limestone. The former river collects, dur- having a mean depth of 2.1 m and a maximum ing its progress, effluent from the moderately depth of 5.5 m, while the former has a mean depth sized towns of Tuam, Dunmore and Ballyhaunis, of 8.4 m. and exceeds46 m in places particularly situated on its tributaries. The secondsub-catch- in the north-eastern and western sides. The ment drains directly into upper L. Corrib. It in- southern and eastern parts of the lake overlie cludesthe river Black, draining from the east,and Carboniferous limestone; in the north and west, the Bealnabrack, Faihnore, Doogtha and the underlying rocks are of schists, granites and Owenriffrivers which drain the mountainous area Silurian rocks, with lower Carboniferous shales to the west and north-west of the upper lake, and sandstonesin the north-eastern comer. In where the soils originate from base-poor 196 Dalradian quartzite, schist and gneiss and also Materials and methods granite parent rocks. Despite their small catch- ments, their contribution to the water budget of Sampling stations the lake is substantial since they drain an area Six sampling stations were chosen on the lake with one of the highestrainfalls in Ireland. Popu- (Fig. 1). Station 1 was located at the south- lation density is low in this areaand upland sheep easternend of the lower lake (M 280 303) where farming is the main agricultural activity. Approxi- water depth is approximately 2 m. During the mately 5% of the areais devotedto forestry. The summer,the lake bottom in this areais coveredby third subcatchmentincludes the areasdrained by a densegrowth of macrophytes.Station 2 (M 276 Loughs Mask and Carra and consists of Car- 335) was sited west of Muckrush Point, wherethe boniferous limestone in the east and Lower water depth is approximately 3 m. Station 3 was Palaeozoic sedimentsin the west. Land useageis situated off Knockferry Point where the lake similar to that in the former two sub-catchments, divides into the upper and lower basins (M 238 depending on the nature of the underlying soil. 420); samplingwas carried out in the centreof the The water from L. Mask, which is 11 m above channel which has a depth of approximately 8 m. that of L. Corrib, leavesvia a natural underground The remaining stations were in the upper basin; channel through the limestone and enters upper station 4 in the south-easternpart (M 160 467) in L. Corrib, near Cong, wherethe channel surfaces. 12 m of water station 5 in the north (M 138 505) This tends to ensurea more or less steadyoutflow in over 30 m of water and station 6 in the north- from L. Mask throughout the year thereby re- western and deepestpart of the lake where water ducing the magnitude of any changes between depth exceeds40 m. summer and winter rates of water flow into L. Corrib. Sampling Water leavesL. Corrib from the southernbasin Sampling was carried out over the period by two routes: through Friar’s Cut, an artificial February 1980to January 1981, the time intervals channel and through the Corrib river (the upper between sampling varying from four to nine section of which is known as ‘The Old River’), on weeks. More frequent sampling did not prove the west. These channelsconverge further down- feasibleon accountof the size and exposednature river, the water then making a rapid descent to of the lake and the frequent strong winds. reach the sea at Galway City. Samplesof surfacewater were taken for physical, Despite its size, no detailed studies have been chemical and chlorophyll analysis in the labora- carried out on the phytoplankton of L. Corrib. tory. In addition, samples were obtained from West & West (1906),Brook (1958)and Round & depths 0,6 and 15 m where thesedepths applied, Brook (1959)made brief qualitative examinations using a water sampling bottle according to of net-haul samplesfrom the lake water. Nutrient Utermdhl. These samples were preserved in and chlorophyll a analysesand qualitative exami- Lugol’s iodine solution for subsequentestimation nation of the phytoplankton were carried out in of phytoplankton cell numbers. From May on- 1973and 1974by Flanagan & Toner (1975), and wards, O-6m integrated samples were also chlorophyll a measurements were made by obtained according to the method described by Champ (1977)in late 1976and early 1977.These Lund (1949) and handled in the same manner. preliminary results indicated that the lake was in Water transparencywas measuredusing a Secchi the mesotrophic or eutrophic-mesotrophiccate- disc. The temperatures of water samples were gory. In the present study, regular sampling over obtained using a mercury thermometer. On a one year period was undertaken to obtain a certain days, temperature profiles of the water more comprehensiveview of the ecology of the column were obtained by measuringthe tempera- lake and in particular, of its phytoplankton. ture of water bottle samplesobtained at 2 m inter- vals down to a depth of 25 m. Qualitative 197 sampling of phytoplankton was carried out using were ahowedto settle in sedimentationchambers a phytoplankton net of mesh size 9.6 x 10- 2 mm and phytoplankton cell numbers estimated using square. an inverted microscope (Utermohl, 1958). In general, the entire bases of the chambers were Water analysis counted but in the case of the taxa Aphanothece Reactive silicate was measuredby a modification sp. and Oscillatoria spp. semi-random fields were of the method givenby Mullin & Riley (1955).The examined. sample was allowed to react with molybdate under conditions which resulted in the formation Lake water levels and waterfrow of a silicomolybdate complex. This complex was Water levels in the lake were obtained from the reducedusing a solution of metol and oxalic acid. records of the Irish Offtce of Public Works. Data The extinction of the resulting blue solution was from continuous level recorders at Annaghdown measuredat 8 12 nm. Solublereactive phosphorus on the lower lake and at Cong on the north shore was measuredby the method outlined by Murphy of the upper lake, were usedin this study. OutfIow & Riley (1962). rates from the lake were calculated using data Nitrate-nitrogen was determinedby a modifica- from a continuous level recorder situated on the tion of the method described in Strickland & Corrib river at Wolfe Tone Bridge, Galway. These Parsons(1972). By passageof the samplethrough data were converted to water flow rates using a cadmium-copper reduction column any nitrate conversionfactors basedon the river’s width and present was reduced to nitrite. The nitrite was tidal flow. Data from 30 rainfall gaugessituated diazatized with sulphanilimide and coupled with throughout the Corrib catchment area(see Fig. 1) N( 1-naphthyl)ethylenediamine to form a magenta were used to calculate the mean daily precipi- coloured azo-dye. The absorbance of this tation in the area and also the percentagecon- compound was measured at 543 nm. Readings tributions by the sub-catchmentsto the total lake were then corrected to take account of nitrite water inflow. present in the original samples. Chlorophyll a was measured by a procedure based on the method of Lorenzen (1967). Pig- Results ments were extracted into cold aqueous 90% acetone. Light extinction was measured at 665 nm. By again measuring light extinction at 665 nm, after acidification of the extract, cor- Physical factors rection was made for phaeopigmentinterference. pH was measuredimmediately using a single Water temperature combined glasselectrode-calomel electrode and a Figure 2 shows the lake surface water tempera- tures at stations 1 and 6. Water temperature digital display meter. Conductivity and alkalinity reached a maximum in late August, the highest were measuredby Burke (1986). Water samples were brought to thermal equilibrium at 20 ’ C and surfacetemperature recorded being 17.5 “C. The surface temperature increased in spring and conductivity then measuredusing a Radiometer declined in winter more slowly at station 6 than CDM3 conductivity meter and type CDC 314 at station 1, the greater volume of water in the electrode of cell constant 0.316cm.