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The microbial ecology of the Bristol Channel
Marine Pollution Bulletin 0025-326X/84 $3.00 + 0.00 © 1984 Pergamon Press Ltd.
Marine Pollution Bulletin, Vol. 15, No. 2, pp. 62-66, 1984 Printed in Great Britain
The Microbial Ecology of the Bristol Channel I. R. JOINT Natural Environment Research Council, Institute for Marine Environmental Research, Prospect Place, The Hoe, Plymouth PL1 3DH, UK Microbial ecology is greatly influenced by the physical processes occurring in the Bristol Channel which is a very dynamic estuary; the large tidal range and tidal excursion maintain high concentrations of inorganic particles in suspension. High turbidity severely limits light penetration, and hence phytoplankton growth, and the large surface area of the particles in suspension influences the growth of bacteria. Other factors which influence the ecology of micro-organisms of the Bristol Channel include the long flushing time and considerable input of dissolved nutrients from the rivers which drain a large area of England and Wales.
Light Limitation of Phytoplankton It is convenient to consider the Bristol Channel as the seven geographical regions shown in Fig. 1. Some regions have very high suspended solids concentrations; in the estuary proper, turbidities of greater than 1000 mg 1-~ have often been measured on flood tide and values of > 100 mg 1 ~are common over most of the Inner Channel. Turbidity decreases in the more seaward regions and typical values for the Outer Channel are < 10 mg 1-j. Turbidity varies with tidal state and with season, being highest on Spring tides at times of maximum ebb and flood. The mean concentration of suspended solids tends to increase in the winter and decrease in the summer (Fig. 2). The high concentrations of suspended solids cause
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rapid attenuation of light in the water; Secchi disc depths vary from l0 m or more in the Outer Channel to less than 10 cm in the estuary (Joint & Pomroy, 1981). The shallowness of the euphotic layer has a considerable influence on phytoplankton ecology in the Bristol Channel; in the Inner Channel, for example, where the mean depth of water is 18 m, the euphotic zone is less than 3°7o of the water column and phytoplankton is severely light-limited and growth might not be expected. However, Joint & Pomroy (1981) estimated the annual primary production in the Inner Channel to be 6.8 g C m 2 y-z and even in the most turbid regions phytoplankton are present and capable of photosynthesis in spite of the apparently severe light limitation. The physical factors which maintain high suspended solids concentrations also result in rapid vertical mixing in the water column. Although the euphotic zone is shallow, all phytoplankton cells in the water column can experience some light because of the intense vertical mixing; the crucial factor, determining whether or not phytoplankton remain long enough in the light to grow, is the time-scale of vertical mixing. Recently, Uncles & Joint (1983) derived time-scales for vertical mixing and discussed the importance of this process to phytoplankton ecology. Phytoplankton, as measured by chlorophyll concentration, is always homogeneously distributed with depth in the water column and vertical mixing is too great to allow the development of high concentrations of phytoplankton cells in the surface,
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Fig. 1 The Bristol Channel showing the seven geographical regions.
62
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Fig. 2 Contour charts of surface suspended solids concentration in March and August 1980. The contours are based on stations corrected to the positions they would occupy at low water; the dotted lines show the sampling tracks, similarly corrected.
euphotic zone. Using estimates of primary production and chlorophyll-a concentration published by Joint & Pomroy (1981) and assuming a carbon:chlorophyll-a ratio of 25, Uncles & Joint (1983) estimated the generation time of phytoplankton in the Inner Channel to be of the order of 70 days. They also estimated the tidally-induced vertical mixing rate in the Inner Channel to be between 1 and 2 h, very much shorter than the phytoplankton generation time. In the Outer Channel, generation times were estimated to be of the order of one day and the vertical mixing time-scale was 4-5 h. So even in the Outer Channel, generation times are longer than the time-scales of vertical mixing and there is no possibility of a chlorophyll maximum developing at any depth. The estimated generation time of 70 days in the Inner Channel suggests that phytoplankton in this turbid region were able to assimilate only marginally more energy by photosynthesis than their maintenance energy requirement; growth, if it occurs, must be very slow. At slow growth rates, advection and dispersion by currents become major factors in determining the spatial distribution of phytoplankton in the Bristol Channel. Uncles & Radford (1980) computed dispersion coefficients and used them to calculate residence time; in the Inner Channel this was of the order of 40 days which is much shorter than the generation time of phytoplankton in this turbid region where advection, therefore, must effectively determine phytoplankton concentration. In localized areas of the Bristol Channel, with shallower water and less tidal dispersion, patches of phytoplankton can develop. For example, Joint (1980) described a patch in Swansea Bay in August 1977, with a chlorophyll-a concentration of 6 mg m -3. Chlorophyll-a was positively
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Fig. 3 The distribution of (a) chlorophyll-a, (b) nitrate, (c) silicate and (d) phosphate on a 5 nautical mile square grid in Swansea Bay in August 1977.
correlated with water temperature and there was a negative correlation between chlorophyll-a and turbidity and between chlorophyll-a and nitrate, phosphate and silicate concentrations (Fig. 3). The conditions of elevated temperature and decreased turbidity resulted in the development of a localized patch of phytoplankton which reduced the concentration of nutrients in the water column. The estimated removal of nitrate in the middle of the phytoplankton patch was 2.76 /ag-at. N 1-1 and of phosphate was 0.16/ag-at. P 1-I. The elemental ratio for the uptake of nutrients was 17N: 1P which compares with the well-known ratio for phytoplankton carbon, nitrogen and phosphorus of 106C: 16N: 1P (Redfield, 1958). Therefore, within Swansea Bay, it is possible for a discrete patch of phytoplankton to develop and be maintained over several days. Collins et al. (1979) described the hydrography of the Bay and found that the main tidal stream ran parallel to the Gower coast with an anti-clockwise eddy within the inner bay; this may have resulted in the partial separation of a water mass in the bay from the adjacent region and allowed the development of the phytoplankton patch. However, the hydrodynamics of Swansea Bay are not known in sufficient detail to distinguish between in situ development of a phytoplankton patch and transport of phytoplankton from other regions.
Primary Production Although the phytoplankton of the Bristol Channel are light-limited, they are unlikely to be limited by nutrients.
63
Marine Pollution Bulletin
Abdullah et al. (1973) reported nitrate concentrations greater than 50 tag-at N 1-1 in the Inner Channel and it is only in the summer that nitrate levels in the Outer Channel are less than 1 tag-at N 1-1. The standing stock of phytoplankton in the different regions is quite low and does not vary much between regions. Chlorophyll-a concentrations range from ca 0.8 mg m -3 in the winter to 1.6 mg m 3 in the summer, although much higher concentrations are found in the nearshore regions; Paulraj & Hayward (1980) reported up to 25 mg m -3 in inner Swansea Bay. In spite of the constancy of standing stock from region to region, there are considerable differences in the rates of primary production. Joint & Pomroy (1981) estimated annual primary production to be 6.8 g C m -2 y r -I in the Inner Channel, 48.5 g C m -2 yr -I in the Central Channel and 164.9 g C m -2 yr -1 in the Outer Channel. The estimated total annual production for each region was 5.2 × 109 g C yr -1 for the whole of the Inner Channel, 8 × 101° g C yr -1 for the Central Channel and 4.7× 1011 g C yr -1 for the Outer Channel. In spite of these very different rates of primary production, phytoplankton biomass does not differ much between the various regions, presumably as a result of grazing in the less turbid regions. In the Outer Channel, the zooplankton is largely herbivorous and presumably consumes a large proportion of the primary production but in the Inner Channel, zooplankton is detritivorous and probably exerts an insignificant grazing pressure on the slow-growing phytoplankton. The characteristic spring blooms of the open seas were not found in the Bristol Channel (Joint & Pomroy, 1981); the highest monthly production occurred in May in the Outer Channel but was one month later in the Central and Inner Channels. There was less seasonal variation in primary production in the Inner Channel than in the other regions, probably as a result of the severe light limitation caused by the high turbidity of the water.
June1971 •
.
.
.
June1974
There were occasional blooms of the colonial Haptophyte, Phaeocystis, but these did not occur every year. Figure 4 shows a contour chart of chlorophyll-a for June 1974 when the concentration of greater than 4 mg m - 3 was due almost entirely to Phaeocystis; the bloom lasted less than one month and in July, chlorophyll-a was less than 2 mg m-3. The contour chart for June 1973 shows the distribution of chlorophyll-a when there was no Phaeocystis bloom (Fig. 4). The factors controlling the development of Phaeocystis blooms are not known but Joint & Pomroy (1981) showed that the response to light of a natural population dominated by Phaeocystis was very different from one in which Phaeocystis was absent. The assimilation number (Pmax) was greater, and occurred at a lower light intensity, than that of a population which did not contain colonial Phaeocystis. So Phaeocystis would appear to be better able to exploit the low light levels which characterize the water column of the Bristol Channel but it is unlikely that this physiological adaptation provides a complete explanation for the development of Phaeocystis blooms.
Phytoplankton Physiology It has been suggested that the rapid vertical mixing experienced by estuarine phytoplankton can result in an alteration of phytoplankton physiology. Demers & Legendre (1982), working in the St Lawrence, observed that increased vertical mixing resulted in a lower assimilation number (Pmax). However, there were no significant variations in the photosynthesis/light curves or assimilation numbers of phytoplankton from the different regions of the Bristol Channel, whatever the time-scale of vertical mixing. It may be that the range in vertical mixing timescales was much greater in the St Lawrence than in the Bristol Channel. Rapid vertical mixing also probably means that natural populations in the Bristol Channel do not experience long periods of intense sunlight and are, therefore, not photoinhibited. Joint & Pomroy (1981) reported that photoinhibition was not normally detected in incubations of less than 2 h; short mixing time-scales will result in movement of phytoplankton cells out of high light intensities at the surface before the photosynthetic mechanism is inhibited by high light. Loss of dissolved organic carbon (DOC) from phytoplankton was generally low; typically between 2 and 4% of the carbon fixed was excreted (Joint & Pomroy, 1981). In the Inner Channel, this would represent an annual input of between 7 and 15 mg C m -3 yr -~ and in the Outer Channel between 90 and 180 mg C m -3 yr-1 and compares with a typical DOC concentration of between 0.8 and 1.6 g C m -3.
Heterotrophic Microbes
Fig. 4 The contoured chlorophyll-a concentration in the absence of Phaeocystis in June 1973 and during a bloom of Phaeocystis in June 1974; all station positions are corrected to high water. Cblorophyll-a expressed as mg chlorophyll-o.m -3.
64
The major source of DOC to the Bristol Channel, however, is riverine; Mantoura & Mann (1979) estimated the total DOC input from rivers varying between 70 t in July and > 2500 t in January. DOC shows a conservative mixing behaviour in the Bristol Channel and a large proportion must be refractory and is not utilized by hetero-
Volume 15/Number2/February 1984
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Fig. 5 Heterotrophicmicrobial activity: distribution of ments in the BristolChannel.
trophic microbes. Joint & Pomroy (1982) studied heterotrophic microbes in the Bristol Channel and found the highest activity in the most turbid regions. Measurements of heterotrophic potential, using 14C-glucose, allowed estimates to be made of maximum uptake rate (Vmax) and turnover time of added glucose. Figure 5 shows the spatial distribution of Vm~xin the Bristol Channel. There was very low activity in the Outer and Central Channels; the highest activity was found in the Outer Estuary and Inner Channel but, at certain times of the year, values of Vmaxwere as low in the Inner Channel as in the Outer regions. A multiple correlation analysis clearly established relationships between Vmax and turbidity (p <0.01), Vm~ and particulate organic carbon (p <0.01), turbidity and salinity (p <0.001), and salinity and POC (p <0.001). There were also weaker correlations (p ~<0.05) between turnover time and number of bacteria (as assessed by plate counts), turnover time and salinity, Vm~ and temperature, and bacterial numbers and temperature. Although there was a strong correlation between heterotrophic potential and turbidity, the highest activity did not always occur at times of highest turbidity. Joint & Pomroy (1982) reported the results of measurements made over a tidal cycle at an anchor station in the Inner Channel. Maximum heterotrophic activity coincided with the lowest salinity water but not with the maximum turbidity; it was not possible to distinguish between turbidity and low salinity water as the factor most influencing microbial activity in the Bristol Channel. The highest numbers of bacteria were found in the low salinity regions of the estuary and ranged from 2.2 × 105 ml -~ in the Outer Estuary to 6 × 102 ml -~ in the Outer Channel. Anson & Ware (1974) and Ware & Anson (1979) reported slightly lower numbers of heterotrophic bacteria but, since they based their estimates on plate counts, the numbers of bacteria were almost certainly underestimated. A study using epifluorescence microscopy is needed before an accurate estimate can be made of the numbers of bacteria in the Bristol Channel.
i
Vmax measure-
Pollution and Microbial A c t i v i t y Anson & Ware (1974) and Ware & Anson (1979) used the numbers of coliform bacteria as indicators of faecal pollution and reported up to 1.8 x 104 coliforms. 100 ml-' seawater off Avonmouth; numbers of coliform bacteria were much lower in the rest of the Bristol Channel and were less than 40 coliform bacteria. 100 ml-] in the Outer Channel. There is a high input of organic matter as sewage effluent to the Bristol Channel but there is no information on the effect of this input on bacterial activity. However, the numbers of bacteria and the heterotrophic potential measurements reported by Joint & Pomroy (1982) do not indicate that the Bristol Channel is severely polluted by organic matter. It has been suggested that high concentrations of heavy metals in the waters of the Bristol Channel might affect phytoplankton production. Davies & Sleep (1979a, b) found that the rate of photosynthesis of phytoplankton taken from the unpolluted waters of the English Channel was depressed by levels of added zinc comparable to those present in the Bristol Channel (Abdullah & Royle, 1974); similar results were obtained with copper (Davies & Sleep, 1980). although the reported levels of zinc and copper in the Bristol Channel are greater than those which inhibit photosynthesis, it is arguable whether heavy metals are limiting phytoplankton production. The turbidity is so great in many areas that phytoplankton is severely lightlimited and heavy metal inhibition may not significantly affect the already limited photosynthesis.
Abdullah, M. I., Dunlop, H. M. & Gardner, D. (1973). Vertical and hydrographic observations in the Bristol Channel during April and June 1971.J. mar. biol. Ass. U.K., 53,557-565. Abdullah, M. I. & Royle, L. G. (1974). A study of the dissolved and particulate trace elements in the Bristol Channel. J. mar. biol. Ass. U.K., 54, 581-597. Anson, A. E. & Ware, G. C. (1974). Surveyof distribution of bacterial pollution in the BristolChannel. J. appl. Bacteriol., 37, 657-661.
65
Marine Pollution Bulletin Collins, M. B., Ferentinos, G. & Banner, F. T. (1979). The hydrodynamics and sedimentology of a high (tidal and wave) energy embayment (Swansea Bay, Northern Bristol Channel). Estuar. cstl mar. Sci., 8, 49-74. Davies, A. G. & Sleep, J. A. (1979a). Photosynthesis in some British coastal waters may be inhibited by zinc pollution. Nature, Lond., 277, 292-293. Davies, A. G. & Sleep, J. A. (1979b). Inhibition of carbon fixation as a function of zinc uptake in natural phytoplankton assemblages. J. mar. biol. Ass. U.K., 59, 937-949. Davies, A. G. & Sleep, J. A. (1980). Copper inhibition of carbon fixation in coastal phytoplankton assemblages. J. mar. biol. Ass. U.K., 60, 841-850. Demers, S. & Legendre, L. (1982). Water column stability and photosynthetic capacity of estuarine phytoplankton: Long term relationships. Mar. Ecol. Prog. Ser., 7, 337-340. Joint, I. R. (1980). Phytoplankton production in Swansea Bay. In Industrialized Embayments and their Environmental Problems. A Case Study of Swansea Bay (M. B. Collins, F. T. Banner, P. A. Tyler, S. J. Wakefield & A. E. James, eds), pp. 469--479. Pergamon Press, Oxford. Joint, I. R. & Pomroy, A. J. (1981). Primary production in a turbid estuary. Estuar. cstl Shelf Sei., 13, 303-316. Joint, I. R. & Pomroy, A. J. (1982). Aspects of microbial hetero-
trophic production in a highly turbid estuary. J. exp. mar. Biol. Ecol., 58, 33--46. Mantoura, R. F. C. & Mann, S. V. (1979). Dissolved organic carbon in estuaries. In Tidal Power and Estuary Management, Proc. 30th Symposium of Colston Research Society (R. T. Severn, D. L. Dineley & L. E. Hawker, eds), pp. 279-286. Scientechnica, Bristol. Paulraj, P. J. & Hayward, J. (1980). The phytoplankton of inshore Swansea Bay. In Industrial Embayments and their Environmental Problems--a Case Study of Swansea Bay (M. B. Collins, F. T. Banner, P. A. "13'ler, S. A. Wakefield & A. E. James, eds), pp. 481--486. Pergamon Press, Oxford. Redfield, A. C. (1958). The biological control of chemical factors in the environment. Am. Sci., 46, 205-221. Uncles, R. J. & Joint, I. R. (1983). Vertical mixing and its effect on phytoplankton growth in a turbid estuary. Can. J. Fish aquat. Sci., 40, (Suppl. 1), 221-228. Uncles, R. J. & Radford, P. J. (1980). Seasonal and spring-neap tidal dependence of axial dispersion coefficients in the Severn- a wide, vertical mixed estuary. J. Fluid Mech., 98, 703-726. Ware, G. C. & Anson, A. E. (1979). The bacteriology of the Severn Estuary. In Tidal Power and Estuary Management, Proc. 30th Symposium Colston Research Society (R. T. Severn, D. L. Dineley & L. E. Hawker, eds), pp. 273-278. Scientechnica, Bristol.
0025-326X/84 $3.00 + 0.00 © 1984 Pergamon Press Ltd.
Marine Pollution Bulletin, Vol. 15, No. 2, pp. 66-70, 1984 Printed in Great Britain
Zooplankton of the Bristol Channel and Severn Estuary R. W I L L I A M S N a t u r a l E n v i r o n m e n t R e s e a r c h Council, Institute f o r M a r i n e E n v i r o n m e n t a l Research, P r o s p e c t Place, The H o e , P l y m o u t h , U K T h e m o s t c o m p r e h e n s i v e set o f d a t a o n t h e z o o p l a n k t o n o f t h e Bristol C h a n n e l c o m e s f r o m t h e m u l t i - d i s c i p l i n a r y s t u d y m o u n t e d b y I M E R . B e t w e e n J u n e 1971 a n d O c t o b e r 1980, a t o t a l o f 1579 n e t h a u l s was t a k e n o v e r a grid o f 58 s t a t i o n s (see Fig. 1). T h e r e w e r e 52 cruises d u r i n g this
p e r i o d ; n o t all s t a t i o n s w e r e visited o n e v e r y cruise b u t e v e r y c a l e n d a r m o n t h , e x c e p t D e c e m b e r , is r e p r e s e n t e d ; most months during the spring and summer seasons being s a m p l e d in five o r m o r e years. T h e r e w e r e o n l y f o u r s t a t i o n s a b o v e t h e H o l m I s l a n d s ; in c o n s e q u e n c e t h e r e is
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Fig. 1 Chart of Bristol Channel and Severn Estuary showing subregions and sampling sites (1-58).
66
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Marine Pollution Bulletin, Vol. 15, No. 2, pp. 62-66, 1984 Printed in Great Britain
The Microbial Ecology of the Bristol Channel I. R. JOINT Natural Environment Research Council, Institute for Marine Environmental Research, Prospect Place, The Hoe, Plymouth PL1 3DH, UK Microbial ecology is greatly influenced by the physical processes occurring in the Bristol Channel which is a very dynamic estuary; the large tidal range and tidal excursion maintain high concentrations of inorganic particles in suspension. High turbidity severely limits light penetration, and hence phytoplankton growth, and the large surface area of the particles in suspension influences the growth of bacteria. Other factors which influence the ecology of micro-organisms of the Bristol Channel include the long flushing time and considerable input of dissolved nutrients from the rivers which drain a large area of England and Wales.
Light Limitation of Phytoplankton It is convenient to consider the Bristol Channel as the seven geographical regions shown in Fig. 1. Some regions have very high suspended solids concentrations; in the estuary proper, turbidities of greater than 1000 mg 1-~ have often been measured on flood tide and values of > 100 mg 1 ~are common over most of the Inner Channel. Turbidity decreases in the more seaward regions and typical values for the Outer Channel are < 10 mg 1-j. Turbidity varies with tidal state and with season, being highest on Spring tides at times of maximum ebb and flood. The mean concentration of suspended solids tends to increase in the winter and decrease in the summer (Fig. 2). The high concentrations of suspended solids cause
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rapid attenuation of light in the water; Secchi disc depths vary from l0 m or more in the Outer Channel to less than 10 cm in the estuary (Joint & Pomroy, 1981). The shallowness of the euphotic layer has a considerable influence on phytoplankton ecology in the Bristol Channel; in the Inner Channel, for example, where the mean depth of water is 18 m, the euphotic zone is less than 3°7o of the water column and phytoplankton is severely light-limited and growth might not be expected. However, Joint & Pomroy (1981) estimated the annual primary production in the Inner Channel to be 6.8 g C m 2 y-z and even in the most turbid regions phytoplankton are present and capable of photosynthesis in spite of the apparently severe light limitation. The physical factors which maintain high suspended solids concentrations also result in rapid vertical mixing in the water column. Although the euphotic zone is shallow, all phytoplankton cells in the water column can experience some light because of the intense vertical mixing; the crucial factor, determining whether or not phytoplankton remain long enough in the light to grow, is the time-scale of vertical mixing. Recently, Uncles & Joint (1983) derived time-scales for vertical mixing and discussed the importance of this process to phytoplankton ecology. Phytoplankton, as measured by chlorophyll concentration, is always homogeneously distributed with depth in the water column and vertical mixing is too great to allow the development of high concentrations of phytoplankton cells in the surface,
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Fig. 1 The Bristol Channel showing the seven geographical regions.
62
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Fig. 2 Contour charts of surface suspended solids concentration in March and August 1980. The contours are based on stations corrected to the positions they would occupy at low water; the dotted lines show the sampling tracks, similarly corrected.
euphotic zone. Using estimates of primary production and chlorophyll-a concentration published by Joint & Pomroy (1981) and assuming a carbon:chlorophyll-a ratio of 25, Uncles & Joint (1983) estimated the generation time of phytoplankton in the Inner Channel to be of the order of 70 days. They also estimated the tidally-induced vertical mixing rate in the Inner Channel to be between 1 and 2 h, very much shorter than the phytoplankton generation time. In the Outer Channel, generation times were estimated to be of the order of one day and the vertical mixing time-scale was 4-5 h. So even in the Outer Channel, generation times are longer than the time-scales of vertical mixing and there is no possibility of a chlorophyll maximum developing at any depth. The estimated generation time of 70 days in the Inner Channel suggests that phytoplankton in this turbid region were able to assimilate only marginally more energy by photosynthesis than their maintenance energy requirement; growth, if it occurs, must be very slow. At slow growth rates, advection and dispersion by currents become major factors in determining the spatial distribution of phytoplankton in the Bristol Channel. Uncles & Radford (1980) computed dispersion coefficients and used them to calculate residence time; in the Inner Channel this was of the order of 40 days which is much shorter than the generation time of phytoplankton in this turbid region where advection, therefore, must effectively determine phytoplankton concentration. In localized areas of the Bristol Channel, with shallower water and less tidal dispersion, patches of phytoplankton can develop. For example, Joint (1980) described a patch in Swansea Bay in August 1977, with a chlorophyll-a concentration of 6 mg m -3. Chlorophyll-a was positively
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15 Nautical miles
Fig. 3 The distribution of (a) chlorophyll-a, (b) nitrate, (c) silicate and (d) phosphate on a 5 nautical mile square grid in Swansea Bay in August 1977.
correlated with water temperature and there was a negative correlation between chlorophyll-a and turbidity and between chlorophyll-a and nitrate, phosphate and silicate concentrations (Fig. 3). The conditions of elevated temperature and decreased turbidity resulted in the development of a localized patch of phytoplankton which reduced the concentration of nutrients in the water column. The estimated removal of nitrate in the middle of the phytoplankton patch was 2.76 /ag-at. N 1-1 and of phosphate was 0.16/ag-at. P 1-I. The elemental ratio for the uptake of nutrients was 17N: 1P which compares with the well-known ratio for phytoplankton carbon, nitrogen and phosphorus of 106C: 16N: 1P (Redfield, 1958). Therefore, within Swansea Bay, it is possible for a discrete patch of phytoplankton to develop and be maintained over several days. Collins et al. (1979) described the hydrography of the Bay and found that the main tidal stream ran parallel to the Gower coast with an anti-clockwise eddy within the inner bay; this may have resulted in the partial separation of a water mass in the bay from the adjacent region and allowed the development of the phytoplankton patch. However, the hydrodynamics of Swansea Bay are not known in sufficient detail to distinguish between in situ development of a phytoplankton patch and transport of phytoplankton from other regions.
Primary Production Although the phytoplankton of the Bristol Channel are light-limited, they are unlikely to be limited by nutrients.
63
Marine Pollution Bulletin
Abdullah et al. (1973) reported nitrate concentrations greater than 50 tag-at N 1-1 in the Inner Channel and it is only in the summer that nitrate levels in the Outer Channel are less than 1 tag-at N 1-1. The standing stock of phytoplankton in the different regions is quite low and does not vary much between regions. Chlorophyll-a concentrations range from ca 0.8 mg m -3 in the winter to 1.6 mg m 3 in the summer, although much higher concentrations are found in the nearshore regions; Paulraj & Hayward (1980) reported up to 25 mg m -3 in inner Swansea Bay. In spite of the constancy of standing stock from region to region, there are considerable differences in the rates of primary production. Joint & Pomroy (1981) estimated annual primary production to be 6.8 g C m -2 y r -I in the Inner Channel, 48.5 g C m -2 yr -I in the Central Channel and 164.9 g C m -2 yr -1 in the Outer Channel. The estimated total annual production for each region was 5.2 × 109 g C yr -1 for the whole of the Inner Channel, 8 × 101° g C yr -1 for the Central Channel and 4.7× 1011 g C yr -1 for the Outer Channel. In spite of these very different rates of primary production, phytoplankton biomass does not differ much between the various regions, presumably as a result of grazing in the less turbid regions. In the Outer Channel, the zooplankton is largely herbivorous and presumably consumes a large proportion of the primary production but in the Inner Channel, zooplankton is detritivorous and probably exerts an insignificant grazing pressure on the slow-growing phytoplankton. The characteristic spring blooms of the open seas were not found in the Bristol Channel (Joint & Pomroy, 1981); the highest monthly production occurred in May in the Outer Channel but was one month later in the Central and Inner Channels. There was less seasonal variation in primary production in the Inner Channel than in the other regions, probably as a result of the severe light limitation caused by the high turbidity of the water.
June1971 •
.
.
.
June1974
There were occasional blooms of the colonial Haptophyte, Phaeocystis, but these did not occur every year. Figure 4 shows a contour chart of chlorophyll-a for June 1974 when the concentration of greater than 4 mg m - 3 was due almost entirely to Phaeocystis; the bloom lasted less than one month and in July, chlorophyll-a was less than 2 mg m-3. The contour chart for June 1973 shows the distribution of chlorophyll-a when there was no Phaeocystis bloom (Fig. 4). The factors controlling the development of Phaeocystis blooms are not known but Joint & Pomroy (1981) showed that the response to light of a natural population dominated by Phaeocystis was very different from one in which Phaeocystis was absent. The assimilation number (Pmax) was greater, and occurred at a lower light intensity, than that of a population which did not contain colonial Phaeocystis. So Phaeocystis would appear to be better able to exploit the low light levels which characterize the water column of the Bristol Channel but it is unlikely that this physiological adaptation provides a complete explanation for the development of Phaeocystis blooms.
Phytoplankton Physiology It has been suggested that the rapid vertical mixing experienced by estuarine phytoplankton can result in an alteration of phytoplankton physiology. Demers & Legendre (1982), working in the St Lawrence, observed that increased vertical mixing resulted in a lower assimilation number (Pmax). However, there were no significant variations in the photosynthesis/light curves or assimilation numbers of phytoplankton from the different regions of the Bristol Channel, whatever the time-scale of vertical mixing. It may be that the range in vertical mixing timescales was much greater in the St Lawrence than in the Bristol Channel. Rapid vertical mixing also probably means that natural populations in the Bristol Channel do not experience long periods of intense sunlight and are, therefore, not photoinhibited. Joint & Pomroy (1981) reported that photoinhibition was not normally detected in incubations of less than 2 h; short mixing time-scales will result in movement of phytoplankton cells out of high light intensities at the surface before the photosynthetic mechanism is inhibited by high light. Loss of dissolved organic carbon (DOC) from phytoplankton was generally low; typically between 2 and 4% of the carbon fixed was excreted (Joint & Pomroy, 1981). In the Inner Channel, this would represent an annual input of between 7 and 15 mg C m -3 yr -~ and in the Outer Channel between 90 and 180 mg C m -3 yr-1 and compares with a typical DOC concentration of between 0.8 and 1.6 g C m -3.
Heterotrophic Microbes
Fig. 4 The contoured chlorophyll-a concentration in the absence of Phaeocystis in June 1973 and during a bloom of Phaeocystis in June 1974; all station positions are corrected to high water. Cblorophyll-a expressed as mg chlorophyll-o.m -3.
64
The major source of DOC to the Bristol Channel, however, is riverine; Mantoura & Mann (1979) estimated the total DOC input from rivers varying between 70 t in July and > 2500 t in January. DOC shows a conservative mixing behaviour in the Bristol Channel and a large proportion must be refractory and is not utilized by hetero-
Volume 15/Number2/February 1984
04~30'
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Fig. 5 Heterotrophicmicrobial activity: distribution of ments in the BristolChannel.
trophic microbes. Joint & Pomroy (1982) studied heterotrophic microbes in the Bristol Channel and found the highest activity in the most turbid regions. Measurements of heterotrophic potential, using 14C-glucose, allowed estimates to be made of maximum uptake rate (Vmax) and turnover time of added glucose. Figure 5 shows the spatial distribution of Vm~xin the Bristol Channel. There was very low activity in the Outer and Central Channels; the highest activity was found in the Outer Estuary and Inner Channel but, at certain times of the year, values of Vmaxwere as low in the Inner Channel as in the Outer regions. A multiple correlation analysis clearly established relationships between Vmax and turbidity (p <0.01), Vm~ and particulate organic carbon (p <0.01), turbidity and salinity (p <0.001), and salinity and POC (p <0.001). There were also weaker correlations (p ~<0.05) between turnover time and number of bacteria (as assessed by plate counts), turnover time and salinity, Vm~ and temperature, and bacterial numbers and temperature. Although there was a strong correlation between heterotrophic potential and turbidity, the highest activity did not always occur at times of highest turbidity. Joint & Pomroy (1982) reported the results of measurements made over a tidal cycle at an anchor station in the Inner Channel. Maximum heterotrophic activity coincided with the lowest salinity water but not with the maximum turbidity; it was not possible to distinguish between turbidity and low salinity water as the factor most influencing microbial activity in the Bristol Channel. The highest numbers of bacteria were found in the low salinity regions of the estuary and ranged from 2.2 × 105 ml -~ in the Outer Estuary to 6 × 102 ml -~ in the Outer Channel. Anson & Ware (1974) and Ware & Anson (1979) reported slightly lower numbers of heterotrophic bacteria but, since they based their estimates on plate counts, the numbers of bacteria were almost certainly underestimated. A study using epifluorescence microscopy is needed before an accurate estimate can be made of the numbers of bacteria in the Bristol Channel.
i
Vmax measure-
Pollution and Microbial A c t i v i t y Anson & Ware (1974) and Ware & Anson (1979) used the numbers of coliform bacteria as indicators of faecal pollution and reported up to 1.8 x 104 coliforms. 100 ml-' seawater off Avonmouth; numbers of coliform bacteria were much lower in the rest of the Bristol Channel and were less than 40 coliform bacteria. 100 ml-] in the Outer Channel. There is a high input of organic matter as sewage effluent to the Bristol Channel but there is no information on the effect of this input on bacterial activity. However, the numbers of bacteria and the heterotrophic potential measurements reported by Joint & Pomroy (1982) do not indicate that the Bristol Channel is severely polluted by organic matter. It has been suggested that high concentrations of heavy metals in the waters of the Bristol Channel might affect phytoplankton production. Davies & Sleep (1979a, b) found that the rate of photosynthesis of phytoplankton taken from the unpolluted waters of the English Channel was depressed by levels of added zinc comparable to those present in the Bristol Channel (Abdullah & Royle, 1974); similar results were obtained with copper (Davies & Sleep, 1980). although the reported levels of zinc and copper in the Bristol Channel are greater than those which inhibit photosynthesis, it is arguable whether heavy metals are limiting phytoplankton production. The turbidity is so great in many areas that phytoplankton is severely lightlimited and heavy metal inhibition may not significantly affect the already limited photosynthesis.
Abdullah, M. I., Dunlop, H. M. & Gardner, D. (1973). Vertical and hydrographic observations in the Bristol Channel during April and June 1971.J. mar. biol. Ass. U.K., 53,557-565. Abdullah, M. I. & Royle, L. G. (1974). A study of the dissolved and particulate trace elements in the Bristol Channel. J. mar. biol. Ass. U.K., 54, 581-597. Anson, A. E. & Ware, G. C. (1974). Surveyof distribution of bacterial pollution in the BristolChannel. J. appl. Bacteriol., 37, 657-661.
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Marine Pollution Bulletin Collins, M. B., Ferentinos, G. & Banner, F. T. (1979). The hydrodynamics and sedimentology of a high (tidal and wave) energy embayment (Swansea Bay, Northern Bristol Channel). Estuar. cstl mar. Sci., 8, 49-74. Davies, A. G. & Sleep, J. A. (1979a). Photosynthesis in some British coastal waters may be inhibited by zinc pollution. Nature, Lond., 277, 292-293. Davies, A. G. & Sleep, J. A. (1979b). Inhibition of carbon fixation as a function of zinc uptake in natural phytoplankton assemblages. J. mar. biol. Ass. U.K., 59, 937-949. Davies, A. G. & Sleep, J. A. (1980). Copper inhibition of carbon fixation in coastal phytoplankton assemblages. J. mar. biol. Ass. U.K., 60, 841-850. Demers, S. & Legendre, L. (1982). Water column stability and photosynthetic capacity of estuarine phytoplankton: Long term relationships. Mar. Ecol. Prog. Ser., 7, 337-340. Joint, I. R. (1980). Phytoplankton production in Swansea Bay. In Industrialized Embayments and their Environmental Problems. A Case Study of Swansea Bay (M. B. Collins, F. T. Banner, P. A. Tyler, S. J. Wakefield & A. E. James, eds), pp. 469--479. Pergamon Press, Oxford. Joint, I. R. & Pomroy, A. J. (1981). Primary production in a turbid estuary. Estuar. cstl Shelf Sei., 13, 303-316. Joint, I. R. & Pomroy, A. J. (1982). Aspects of microbial hetero-
trophic production in a highly turbid estuary. J. exp. mar. Biol. Ecol., 58, 33--46. Mantoura, R. F. C. & Mann, S. V. (1979). Dissolved organic carbon in estuaries. In Tidal Power and Estuary Management, Proc. 30th Symposium of Colston Research Society (R. T. Severn, D. L. Dineley & L. E. Hawker, eds), pp. 279-286. Scientechnica, Bristol. Paulraj, P. J. & Hayward, J. (1980). The phytoplankton of inshore Swansea Bay. In Industrial Embayments and their Environmental Problems--a Case Study of Swansea Bay (M. B. Collins, F. T. Banner, P. A. "13'ler, S. A. Wakefield & A. E. James, eds), pp. 481--486. Pergamon Press, Oxford. Redfield, A. C. (1958). The biological control of chemical factors in the environment. Am. Sci., 46, 205-221. Uncles, R. J. & Joint, I. R. (1983). Vertical mixing and its effect on phytoplankton growth in a turbid estuary. Can. J. Fish aquat. Sci., 40, (Suppl. 1), 221-228. Uncles, R. J. & Radford, P. J. (1980). Seasonal and spring-neap tidal dependence of axial dispersion coefficients in the Severn- a wide, vertical mixed estuary. J. Fluid Mech., 98, 703-726. Ware, G. C. & Anson, A. E. (1979). The bacteriology of the Severn Estuary. In Tidal Power and Estuary Management, Proc. 30th Symposium Colston Research Society (R. T. Severn, D. L. Dineley & L. E. Hawker, eds), pp. 273-278. Scientechnica, Bristol.
0025-326X/84 $3.00 + 0.00 © 1984 Pergamon Press Ltd.
Marine Pollution Bulletin, Vol. 15, No. 2, pp. 66-70, 1984 Printed in Great Britain
Zooplankton of the Bristol Channel and Severn Estuary R. W I L L I A M S N a t u r a l E n v i r o n m e n t R e s e a r c h Council, Institute f o r M a r i n e E n v i r o n m e n t a l Research, P r o s p e c t Place, The H o e , P l y m o u t h , U K T h e m o s t c o m p r e h e n s i v e set o f d a t a o n t h e z o o p l a n k t o n o f t h e Bristol C h a n n e l c o m e s f r o m t h e m u l t i - d i s c i p l i n a r y s t u d y m o u n t e d b y I M E R . B e t w e e n J u n e 1971 a n d O c t o b e r 1980, a t o t a l o f 1579 n e t h a u l s was t a k e n o v e r a grid o f 58 s t a t i o n s (see Fig. 1). T h e r e w e r e 52 cruises d u r i n g this
p e r i o d ; n o t all s t a t i o n s w e r e visited o n e v e r y cruise b u t e v e r y c a l e n d a r m o n t h , e x c e p t D e c e m b e r , is r e p r e s e n t e d ; most months during the spring and summer seasons being s a m p l e d in five o r m o r e years. T h e r e w e r e o n l y f o u r s t a t i o n s a b o v e t h e H o l m I s l a n d s ; in c o n s e q u e n c e t h e r e is
o
oi INNER
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Fig. 1 Chart of Bristol Channel and Severn Estuary showing subregions and sampling sites (1-58).
66
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