MEDIDAS DE LONGITUD

In contrast to the varied seasonal distributions of P in the studied water bodies, NO3 showed a remarkably consistent seasonality across the entire data-set, which conforms to the patterns commonly observed in temperate lakes (Wetzel, 1983). Most of the studied sites experience late winter/early spring maxima, followed by a period of low concentrations through summer and autumn, and a gradual increase again in the next winter. This seasonal pattern of events is usually attributed to increased rates of NO3' assimilation by algae throughout the spring and summer growing season, and has been observed in many studies (eg. Moss, 1969; Wilson et al.,

1975; Gibson et al., 1980; Reynolds, 1973b; Bailey-Watts et al., 1987, 1990).

Denitrification is also an important process, particularly in eutrophic lakes where anaerobic conditions often prevail in summer, whereby bacteria biochemically reduces the oxidized N anions resulting in a loss of N from the system if it is not refixed (Wetzel, 1983). This process has been found to contribute significantly to N removal in a number of shallow, eutrophic lake studies (eg. Anderson, 1974; Johnston et al., 1974; Bailey-Watts et al., 1987; S<|)ndergaard et al., 1990a; Jensen et al., 1992a). For example, denitrification was responsible for 86-93% of the net N loss in shallow, hypertrophic Lake S<|)bygaard (Jensen et a l, 1992a). This process was stimulated by resuspension and short retenticxi time, as O2 and NO3 were introduced into the deeper sediment layers, thus enhancing denitrification activity at sediment depth. A further possible explanation for the NO3 decrease in spring and summer is removal of NO3 from the epilimnion to the sediment via N uptake by phytoplankton and sedimentaticm of organic matter, which is known to take place in shallow productive waters where the sedimentation rate is often high (S<j)ndergaard et al., 1990a). For example, in a small, eutrophic lake in Ontario, phytoplankton were primarily responsible for NO3 uptake in the epilimnion (Chan & Campbell, 1980). In the winter NO3 concentrations increase, as uptake by phytoplankton and denitrifying bacteria is minimal. Weather conditions may also be important as NO3 normally enters water from diffuse agricultural sources. Bailey-Watts et al. (1990) showed that high flushing rates resulted in high winter NO3 concentrations in Loch Leven.

There are only two sites in the data-set that do not display this typical seasonal pattern in NO3

concentrations. Both of these sites maintain high concentrations throughout the study period, never falling below 2.0 mg 1^ at site No. 37, and never below 1.3 mg 1*^ at site No. 57. Both of these ponds receive agricultural run-off, which is the most likely source. NO3 does not appear to be limiting at any of the 31 water bodies, as concentrations over the year never fall below 0.3

mg at any site. In summary, the shallow waters in south-east England appear to behave in the same way as many other shallow, j»*oductive lakes, in terms of their seasonality of NO/ concentrations.

4.3 3.3 Dissolved silica (SiOg)

There is a clear seasonality in SiOg concentrations in the studied water bodies, which conforms to the general pattern observed in temperate lakes (Wetzel, 1983). This is a pattern of spring and often autumn minima, with replenished concentrations in the intervening periods. This pattern emerges because of the close association between diatom growth phases and the availability of SiOg, such that a spring decline in SiOg is often coincident with the observed diatom maxima as the diatom cells assimilate large quantities of this nutrient. This phenomenon has been observed in the southern basin of Lake Windermere in the English Lake District for 18 consecutive years (Lund, 1964) and in numerous other enriched lakes (eg. Moss, 1969; Happey, 1970; Reynolds 1973b; Gibson & Stevens, 1979; Hecky et al., 1986; Bailey-Watts & Lund, 1973; Bailey-Watts, 1976a, 1976b, 1988) and in rivers (eg. Lack, 1971).

The patterns in the south-east England waters are generally in accordance with these observations, although given that the sampling frequency was at monthly intervals and chlorophyll a provides the only estimate of phytoplankton biomass (which is not necessarily dominated by diatoms), it is not possible to directly relate SiOg concentrations to diatom maxima. SiOg is also taken up by non-planktonic diatoms, whose biomass is not accounted for in the chlorophyll a measurements. However, peaks in chlorophyll a concentration coincide with SiOg decline at a number of sites, for example Nos 4, 7, 37, 65, 69, 73, 79, 100 and 120. SiOg concentrations decreased during the spring or early summer at all sites, and similarly to many eutrophic lakes, were depleted at this time. Concentrations fell below 0.5 mg 1^ on at least one occasion (cf. Kilham, 1975) at all sites except Nos 34, 57, 86 and 107, suggesting that SiOg may be limiting in many of these waters.

Generally, SiOg concentrations rose again in the summer as the supply was replenished, either as a result of reduced biological utilization from catchment inputs, or via recycling, release of silica from dissolution and sediment release, or a combination of these factors. The possible influence of sediment release of SiOg on summer concentrations was discussed earlier in 4.3.2.6. The mild, calm weather and low rainfall in the summer of 1991 suggests that flushing rate was probably relatively low at that time, and so SiOg recycling could have been enhanced (Bailey-

Watts et al., 1990). Periods of temporary stratification in even the shallowest waters occur (4.3.2.9) when diatoms would likely sink quite quickly to the sediments. Subsequent dissolution of the diatom frustules would provide a significant source of SiOg to the sediments, which in turn can be transported back into the water column in the summer if the conditions favour nutrient release.

A study of a one month silica budget in Loch Leven (Bailey-Watts, 1976b) highlighted the importance of the diatom frustule as a source of silica, even though, as in the south-east England ponds, the short water column and generally good preservation of diatoms in the sediments indicated that dissolution was not important The study demonstrated that only slight erosion of cells or complete dissolution of only certain cells could allow the observed abundance of diatoms in sediment cores (cf. Haworth, 1972) as well as a significant degree of silica recycling. Therefore, although minimal dissolution was observed in the south-east England surface sediments, this does not necessarily preclude the importance of diatoms in the silica cycle. The similar timing of summer peaks in TP and SRP at a number of sites lends further support to the theory that internal loading contributed to the rapid rise in SiOg concentrations during the summer (Osborne & Phillips, 1978).

4.3.3.4 Chlorophyll a

There was no overall seasonal pattern in chlorophyll a concentrations in the data-set. This was expected, as the literature emphasises the considerable variation in biomass maxima between years at any one site, and reports that widely fluctuating algal crops are commcwi in temperate environments (eg. Tailing, 1965; Bailey-Watts, 1982). Chlorophyll a concentrations can also change rapidly within a single year; for example, in hypertrophic Lake S(J)bygaard (Jeppesen et at., 1990a), chlorophyll a concentrations changed from 1200 pg 1*^ to 650 pg 1^ in a period of 20 days, and then fell sharply to 12 pg 1'^ within a further 3-5 days, owing to a severe phytoplankton collapse in July 1985. This was followed by a rapid recovery 3-4 days later. Given this within-site variability of phytoplankton biomass from year to year, a uniform pattern between sites was unlikely to be observed, and chlorophyll a seasonality was expected to be site specific (cf. Reynolds, 1984a). A whole variety of factors such as grazing pressure (Uhlmann, 1971; Leah et al., 1980; Vanni & Temte, 1990), fungal parasitism (Canter & Lund, 1953, Bailey-Watts & Lund, 1973), flushing rate (Bailey-Watts et al., 1990), degree of stratification (Happey, 1970; Reynolds, 1973b; Stauffer, 1988), and sedimentation rate (Gibson et al., 1971; Jewson et al., 1981; Gibson, 1984) can influence the seasonality of phytoplankton and hence

chlorophyll a concentrations at a given site.

Despite the variability between sites, a high incidence of similarity among periodic cycles in phytoplankton biomass of similar lake types has been observed. For example, Reynolds (1984a) classified phytoplankton species into 14 groups, in CM*der to characterize periodicities for five major lake types based on trophic status. However, he reported an unfortunate lack of information for "ponds and shallow lakes subject to high nutrient loading" but proposed a typical seasonal pattern nonetheless. Reynolds (1984a) suggested that biomass was usually dominated by diatoms, euglenoids or chlorococcalean genera in these types of waters, and that assemblages were abundant in early spring, some of which could persist throughout the year.

Alternatively, a progression frcxn diatom-dominated assemblages in spring to cyanobacteria in late summer/autumn has also been observed in shallow, enriched lakes (Gibson et al., 1980; Reynolds, 1984a) and in rivers (Lack, 1971). Diatoms often dominate the phytoplankton in spring because of their photosynthetic efficiency, allowing them to grow under conditions of low light and temperature, relative to other algal types (Lund, 1965). A second diatom peak is often observed in association with destratification in autumn (Reynolds, 1984b). However, in the mid­ summer period, nutrient limitation prevents the development of high biomasses or may favour the dominance of blue-green algae that can out-compete diatoms under these conditions. This pattern has been observed in numerous productive lakes, deep enough to stratify [eg. Happey (1970), Reynolds (1973b, 1978b), and Vanni & Temte (1990)]. However, in well-mixed, very shallow, enriched lakes, where stratification may only last for a few hours or days, and/or where nutrients may never become limiting, as in the south-east England ponds, this pattern is less likely to be observed (Reynolds, 1973c).

Twelve of the studied south-east England ponds have maximum chlorophyll a concentrations in summer (June to September) and six sites have maxima in spring (March to May) with generally low concentrations in winter and smaller peaks throughout the rest of the year. A number of sites have both a spring and autumn peak in chlorophyll a concentrations with a period of lower concentrations in mid-summer (eg. Nos 57, 73, 100, 112, 113). Therefore, some sites largely conform to the commonly observed chlorophyll a seasonality in other shallow, eutrophic lakes. However, no distinct seasonality can be recognized at the remaining sites.

Though it is not possible to determine the limiting factors controlling the chlorophyll a concentrations at each individual site, P and Si02 limitation are likely to be more important than N limitation in the sampled waters. For example, SRP could be limiting at some sites (ie. < 5 pg r^), where concentrations are only just detectable following a peak in chlorophyll a concentrations. For example, at site No. 76 where SRP fell from 9.79 pg 1^ in August 1991 to 2.64 pg 1^ in September, and further declined to 1.58 pg 1^ by October. At this site chlorophyll a reached a maximum of 159.36 pg 1^ in August and then sharply declined to 42.72 pg 1^ by September. This suggests that the available P was rapidly assimilated by the algae, until SRP concentrations were reduced to levels too low to promote further growth and hence a collapse occurred.

Alternatively, Si02 could be limiting for diatoms in some cases, where minimum concentrations are associated with maximum chlorophyll a concentrations. For example, at site No. 65, Si02 concentrations were reduced from 2.34 mg 1'^ in May to only 0.002 mg 1^ in August, coinciding with the maximum chlorophyll a concentration of 233.28 pg l'\ However, SiÛ2 concentrations generally rose quickly following depletion and poor diatom preservation was rarely observed in the surface sediments (see 4.3.3.3), indicating that concentrations may have been low enough to limit diatom production for only very short periods. NOg does not appear to be limiting at any site, as concentrations are always in excess of 0.3 mg 1'^ (see 4.3.3.2). At many sites, none of the nutrients ever drop to concentrations low enough to limit growth and so other factors such as predation, parasitism or light could be controlling phytoplankton biomass. The probable occurrence of sediment resuspension at some sites (see 4.3.3.1) would certainly influence light penetration.

The role of seasonally changing nutrient ratios, particularly Si:P ratios (eg. Kilham, 1971; Tilman, 1977; Stauffer, 1986; Hecky & Kilham, 1988) and N:P ratios (eg. Redfield, 1958; Rhee, 1978; Smith, 1982, 1983; Barica, 1990; Molot & Dillon, 1991; Sommer, 1991) in determining nutrient limitatim, and their impact on phytoplankton populations has received a great deal of attention. For example, Redfield (1934, 1956) proposed that the ideal atomic ratio of the key nutrients to supply cell material for the next generation in seawaters was 106C:16N:1P. These ratios apply also to freshwaters (Sommer, 1991). This ratio can be used as an indication of N or P limitation, whereby < 16N:1P indicates N limitation, and > 16N:1P indicates P limitation. Smith (1983) demonstrated that the relative proportion of blue-green algae in the phytoplankton was dependent on the N:P ratio, with low ratios favouring blue-green algal blooms as these algae

are capable of N fixation. However, there are a number of (xoblems associated with N:P ratios. For example, Rhee (1978) showed that the optimal cellular ratio was species specific, Molot and Dillon (1991) demonstrated that the ratio could not always explain the variation in chlwophyll a, and moreover, there is no uniformity in the forms used to calculate N:P ratios in the literature, making comparison betweoi studies problematic (Barica, 1990).

In the south-east England study, TN was not measured and therefore tiie importance of the N:P ratio in these waters cannot be fully explored. However, atomic ratios of NOj'iSRP (as N and P) were calculated for a number of sites to explore the overall patterns in the data. At site No. 31 and No. 120, where SRP and Si02 were in high concentrations throughout the year, the N:P ratio was approximately 3:1 in both winter and summer, and 5:1 in winter and 3:1 in summer respectively, indicating NO/ limitation even though concentrations never fell below 0.35 mg 1^ at these sites. In a study of the Cheshire and Shropshire meres (Moss et aL, 1993), where TP concentrations are very high but where winter NO3' values are generally low (< 2 mg 1'^), similarly low N:P ratios (calculated from winter means of inorganic N and SRP) of < 10:1 (by weight) were reported. At site No. 37, where NO3 concentrations were high throughout the year, the N:P ratio was 24:1 in winter and 42:1 in summer, indicating P limitation during the growing season. Therefore, the ratio can provide a guide to nutrient limitation at these sites, though it has been suggested that in reservoirs and shallow waters where particulate P is relatively important, that chlorophyll a may be affected more by variations in suspended matter and turbidity than N:P ratios (Smith, 1982).

SiOg is likely to be limiting for only short periods in most of the south-east England ponds, as previously discussed. Small, shallow lakes generally tend to have high Si:P ratios (Kilham & Kilham, 1990). However, atomic ratios of Si02:SRP (as Si and P) were calculated for a number of ponds to allow comparison between sites. Site No. 65 has a high winter ratio of 7:1, but in summer when SiOg concentrations drop to 0.002 mg l'\ the ratio is < 0.1:1. Site No. 31 displays a similar pattern, with a very high ratio in winter when both SiOg and SRP are high of 32:1, but a ratio of only 3:1 in March, when SRP concentrations remain high but SiOg concentrations fall to 0.01 mg r \ At site No. 113, where SRP concentrations are relatively low, the Si:P ratio is > 25:1 throughout the year. In the absence of phytoplankton data, and given the seasonal variability in ratio values, it is difficult to interpret these data. However, if the annual mean Si:P values for these three sites are calculated (No. 65 = 5:1; No. 31 = 15:1; No. 113 = 40:1) and the dominant diatom taxa in the surface sediment assemblages of these sites are considered

(No. 65 = Cyclotella meneghiniana; No. 31 = Fragilaria construens var. venter; No. 113 =

Stephanodiscusparvus and Asterionella formosa; see Chapter 5), the patterns are consistent with

those reported in the literature [Chapter 1 (1.2.2)] where C. meneghiniana is dominant at low Si:P ratios, and Æ formosa is dominant at high Si:P ratios (Kilham & Tilman, 1979). The role of Si:P ratios in controlling diatom distributions is discussed in more detail in Chapter 7 (7.2.2).

It is not possible to provide an explanation for the observed patterns in chlorophyll a concentrations in the studied sites, given that the live phytoplankton populations were not monitored. Chlorophyll a was measured simply to provide some estimate of lake productivity rather than to investigate phytoplanktcm periodicity (cf. White et al, 1988). However, the data do serve to highlight the complexities of these shallow productive waters and demonstrate the ways in which tiiey differ from lakes that are deep enough to stratify for long periods. The calculations made here indicate that nutrient ratios may be important in these shallow waters, and if the data-set were expanded in future to include less productive sites, the roles of SiOg and NO) limitation could be more fully explored.