THE IMPACT OF ACIDIFICATION ON DIATOMS AND CHEMISTRY OF DUTCH MOORLAND POOLS

HERMAN VAN DAM

(Research Institute for Nature Management, P.O.Box 46, 3956 ZR Leersum, the Netherlands)

GI LLIS SUURMOND

(Nature Conservation Department, Agricultural University, P.O.Box 8080, 6700 DD Wageningen, the Netherlands)

CAJO TER BRAAK

(Institute T.N.O. for Mathematics, Information Processing and Statistics, P.O.Box 100, 6700 AC Wageningen, the Netherlands)

(SUMMARY)

Moorland pools are shallow oligotrophic soft water lakes on poorly buffered sandy mils. Diatom assemblages of samples from 16 pools taken in 1920 and 1978 ware compared by analysis of pH-spectra, diversity, dissimilarity and multivariate statistical techniques.

The pH-spectra of pools in the southern (S) and central (C) part of the country indicate a fall in pH from 4.5 o 6.0 in the old samples to 3.7 - 4.6 in the recent ones. The pH-spectra of the northern pools (N) do not indicate a significant shift from the original pH (ca 4.5).

The number of species in the count and the diversity (indices of Simpson and Shannon) decreased significantly in S-I-C, and that goes also for the dissimilarity index of Dyer. No changes were found in N.

The first component (PC 1) of the principal component analysis explains 61% of total variance. PC 1 is correlated with log ESO4~ (r -- 0.83, p < 0.001) and even better (r = 0.95, p < 0.001) with the relative sulphate concentration, Le. the ratio of sulphate to all major anions (sulphate, chloride, bicarbonate). All old samples have low scores on PC 1, recent samples have low scores on the second (PC 2) and third (PC 3) principal component. Old samples have high scores on PC 2 and PC 3, explaining 9 and 6% of total variance, respectively. The orginal variation, caused by regional factors, is replaced by a SO42"controlled variation. PC 1 is nearly completely determined by the relative abundance of Eunotia exigua. "This species, which is known to be very resistant to pollution by sulphur, aluminium and heavy metals, increased largely from 1920 to 1978.

In spite of the rather homogeneous distribution of wet sulphate deposition in the Netherlands, . substantial differences in SO42"content in the pools are observed, being lowest in N (O.13 - 0.48 meq.I "1) and highest in S+C (0.38- 1.65 meq.l" 1 ). Sulphate is positively correlated with calcium, aluminum and magnesium but negatively with factors that characterize humic acid waters (e.g. permanganate-consumption, iron and the ratio of univalent to divalent cations). Sulphate concentration depends on the intensity of sulphate reduction, accumulation by dry deposition in surrounding forests of Scots pine, drought and atmospheric deposition.

DIEL PATTERNS OF PHYTOPLANKTON PRODUCTIVITY IN LAKE VECHTEN

W.OPSTELTEN

(Limnological Institute, Vijverhof Laboratory, Nieuwersluis (U.), The Netherlands)

(SUMMARY)

A review of the literature on phytoplankton primary production studies in lakes indicates that in most cases the daily primary production rates have been derived by multiplying the short-term (usually 3 or 4 hours) production measurement with a light factor. This factor is a ratio between the total irradiance during the day and during the period of incubation. The aim of the present study was to verify this method of calculation. Since the primary production rate is of great importance for the carbon cycle of lakes, this study can be regarded as a fundamental contribution to the integrated research of the ecosystem of lake

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Vechten (the Netherlands) (BEST et el., 1978). Photosynthetic carbon fixation was determined using the radiocarbon method of STEEMANN

NI E I~EN ( ! 952). The in $itu measurements were carried out for 2 hours incubation periods starting at 15-30 minutes before sunrise and ending 15-30 minutes after sunset. Also measured ware: water temperature, light penetration, oxygen and chlorophyll. Total incident solar radiation data ware obtained from the Royal Netherlands Meteorological Institute (K.N.M.I.) at de Bilt (situated at 5 km from lake Vechtan). The production during two consecutive 2-hourly periods, e.g. period 1 and period 2, were summed up and using the irradiance data during the 4 hours, daily production was calculated. These calculations were repeated for period 2 and 3, and so on. The sum of all the short-term measurements during a day, i.e. the total daily primary production, was taken for comparison with the production calculated by using the sum of the production during two successive periods and the light factor.

I I I I I I I %

1=o- V E C H T E N 1 9 7 9

A Pcml = prod. �9 vie short / 100- q exp. time and light /

8 0 - ~ P , . = . p. diurnal . x p o s u r . / \

eo-~ ~"

eL + 40 -

I~l. 2 0 -

e ~176 "'~176176 o """

~ 2 o ~ "'~ "%~176 " ""

a 8 10 12 14 16 18 T I M E

Fig. 1. Daily production rate calculated (Pcal) from the 2-hourly production measurements and the daily solar radiation as per cent of the integral production (Pint) obtained by summing up the 2-hourly production measurements during the course of the day.

Daily production rates obtained by extrapolating from short-term incubation periods both in the morning and in the late afternoon appeared to overestimate the 'real' daily production (Fig. 1 ) up to 100%. It is likely, that in the morning the lack of nutrient limitation and relatively better physiological state of algae, and in the afternoon increase of phytoplankton biomass during the day can be considered as the factors causing the discrepancy between the calculated and 'real' production. On the other hand, the underestimation of daily production rate based on midday time intervals on sunny days, was about 25%. It is attributed to photoinhibition, especially in the present study where the measurements ware limited to the upper water strata ((~4m).

Is is concluded that short-term in situ exposures between 10.00 and 14.00 hr appeared to be a good indicator of total primary production, although underestimates up to 25% on sunny days are to be taken into account.

REFERENCES

BEST, P.H., MARIANNE C.I. BLAAUBOER, Th.E. CAPPENBERG etaL, 1978. Towards an integrated study of the ecosystem of lake Vechten. HydrobioI.BulI.Amsterdam, 12:107-118.

STEEMANN NIELSEN, E., 1952. The use of radio-active carbon (C 14) for measuring organic production in the sea. J. Conseil, 18:117-140.

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