Hydro-ecological relations in the Delta Waters

occwrence of floating mats of blue-green algae often causes a nuisance
in tMs lake. The mean transparency does not exceed 1 m.
Figure 6 shows the changes in fhe chlorophyl level in bofh parts of
the lgke system in 1988.
' Laks
VoLsrak
---- Lake zom
-
0 6 TO 9 6 20 26 30 85
week no.
Figure 6 Chlorophyl level id take Volkerak and lake Zoom in 1988
From this Figure it can be seen that peaks of up tn 100 mg/m9 o a r tn
the s~ring. There fs an abrupt downturn at Che end of &lax/he~inning of
June, and fn the reat of the summer the chlorophyl levels remain very
low - especially in Lake Volkerak.
The next two Sigures (Fig. 7 and 8) show the changes in the total
P-level and the transparency.
There was a clear decrease is the level of total pho~gharrrs in the
spring. In the perirvd of the sharp deeIine in the ehlorophyl level
towaras the end of May there is a clear increase in traneparency, first
in Lake Polkerak and later Ln Me Zoom too. In hrvth lakes a
transparency of alnurst 4 m was measured in August - a unique situation
fos fresh water in the Netfierlds.