Figure 6. Cluster analysis of microplankton samples (dates) from Coliumo Bay between 11 and 30 April 1997 (one sample per day). Analysis was based on a Bray-Curtis similarity matrix of taxa abundance. Dotted line indicates reference value used to separate groups (average of Bray-Curtis similarity matrix).
Figure 4. Chlorophyll-a (A) and phaeopigments (B) concentrations (µg L-1) measured at Coliumo Bay for the period 11th – 30th April 1997. Dots indicate sampling dates and depths. On April 22nd (storm) only surface and bottom samples were taken. Note that interpolation during the storm and after-storm period was kept to a minimum since no information is avaliable on pigment change patterns on a daily scale.
Wind intensity had detrimental direct effects on chl stratification as indicated by negative correlation between both variables. But a short period of N wind with sustained velocities up to 40 km h-1 had delayed positive effects that favoured phytoplankton blooming (sensu Sverdrup 1957) and stratification. Strong stratification was a short lived phenomenon (< 4 d), but post-storm biomass levels roughly 10 – 20 µg L-1 constitute remarkably high values for this site (Augsburger 1981, Calliari 1999) and lasted for at least twice as long (22 – 30 April). It is unlikely that the observed chl increase resulted from an intrusion of high chl waters from adjacent areas. No estimates outside Coliumo Bay are available for the same period, but chl levels outside the bay are generally much lower than those reported here for the post-storm period (Augsburger 1981, Peterson & Bellantoni 1987, Peterson et al. 1988).
It could be argued whether chl increase during storm and post-storm periods reflected real phytoplankton growth, or mainly resulted from turbulence-induced resuspension of sedimented, senescent cells. Phaeopigments could be of use as tracers of resuspended, partially decomposed phytoplankton-derived matter; they indeed increased
Figure 5. A.- Plot of chlorophyll stratification index S vs. wind velocity in Bahia Coliumo for the period 11 – 30 April 1997. B - Time series of chlorophyll stratification index S for the same period. ♦= S estimated 48 hs after storm (April 26th), for details see text.
resuspension that may also explain the quick chl increase between April 20th and 23rd (storm period). However, active growth likely contributed to the biomass peak following the storm as indicated by continued increasing trend of chl to maximum levels after the storm, while phaeopigments disappeared from the water column (presumably due to sinking). The fact that highest chl concentration and its maximum stratification occurred simultaneously further suggests that both processes were mediated by algal growth rather than by turbulent mixing.