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Short-term changes in the concentration of chlorophyll.

19

Changes in the microplankton assemblage also support that biomass enhancement was mostly mediated by resuspension of benthic micro algae (during the storm) and by active growth (after the storm). The diatoms Amphora sp., Cymbella sp., Cocconeis sp., Paralia sp. and Pinnularia sp. only occurred during the storm. Paralia is a coastal brackish genus frequently found in the sediments and water column at shallow coastal systems (Witkowski et al. 2000). Cymbella, Amphora and Pinnularia are mostly freshwater/brackish epibenthic diatoms which may be resuspended during highly energetic conditions (Krammer 2000, 2002, Krammer & Lange-Bertalot 1988, 1991a, 1991b, Metzeltin & García-Rodríguez 2004), so their presence in the plankton strongly suggests resuspension processes contributing to bulk algal biomass in the water column during the storm. Several taxa increased their numbers after the storm, but Eucampia sp. showed the most remarkable shift from 102 cells L-1 to 106 cells L-1 over a 10 d period. Also, cell numbers after the storm considering all taxa were higher than before storm numbers by a factor > 50, a further indicator that algal growth was an important factor contributing to the observed biomass increase. Among the least abundant groups is noteworthy that ciliates and tintinnids, representing the heterotrophic components of the microplankton, showed an order of magnitude increase in their abundance during and after the storm compared to before storm values.

Interestingly, vertical stratification of chl also peaked ca. 48 hs after the storm along with phytoplankton biomass, a somewhat counterintuitive finding. Formation of SCM can respond to a combination of physical, physiological and behavioural mechanisms. Briefly, physical factors involve the formation of pycnocline and nutricline, and the depth distribution of light. Physiological features may include the growth response to the physical scenario, like shifting the carbon:Chl ratio

(shade

adaptation,

Steele

1964).

Behavioural

responses include aggregations at preferred depth ranges mediated by vertical migration (mostly by dinoflagellates but also diatoms, Iriarte & Bernal 1990). Relative importance of each mechanism may vary from one geographic area to another (Cullen & Eppley 1981). It is beyond the scope of this paper to identify actual mechanisms leading to observed changes in chl concentration and vertical distribution. However, we hypothesize that phytoplankton growth was likely enhanced by a pulse of nutrients supplied by the strong mixing of the water column. It should also be noted the potential improvement in the light environment (i.e. mixed to photic depths ratio) experienced by

diatoms due to resuspension by increased turbulence and a subsequent stabilisation of the water column after the storm. Indeed, stabilization may have been favoured by freshwater runoff after the storm, although lack of CTD data for this period prevents to assess this matter. This interpretation is consistent with results from deeper areas: discrete wind- induced mixing events also led to nutrient enrichment that stimulated subsequent phytoplankton growth and biomass accumulation in the euphotic zone (Marra et al. 1990, Kiørboe & Nielsen 1990, Nielsen & Kiørboe 1991). Results of mesocosms studies that assessed combined effect of mixing and nutrient addition suggested similar patterns (Donaghay & Klos 1985, Estrada et al. 1988).

Possible consequences of severe wind events for higher trophic levels are not obvious, since they represent the outcome of relatively complex trophic processes. On one side, short-lived blooms may not be efficiently transferred to higher trophic levels. The fraction of energy transferred to large consumers by trophic interactions is dependent on the dominant grazer type (i.e. metazoan vs. protozoan; Kiørboe 1993), and the match between the time scale of the phytoplankton bloom and the time response of their grazers (Kiørboe & Nielsen

  • 1990)

    . Ephemeral pulses of enhanced production

    • (i.

      e. lasting few days) may not be usable by

metazoan grazers like copepods to increase population numbers (Mann & Lazier 2006). For example, small coastal copepods (i.e. genus Acartia,

or

Centropages)

peaks

associated

Paracalanus

may exhibit

productivity

to short-lived

phytoplankton blooms, but their relatively long generation times (ca. one month) prevent population increases on a daily or weekly scale (Kiørboe & Nielsen 1990). In contrast, ciliated protozoa and parthenogenetic metazoans (rotifers) have higher growth rates and much shorter generation times (hours to days) enabling faster numerical responses to sudden changes in food availability (Nielsen & Kiørboe 1990), as observed here for heterotrophic protozoans. However, the production of such organisms may not be readily available for higher

trophic levels. On the other side, phytoplankton and protozoan

actively growing microplankton (as

observed in post-storm conditions) represent highly nutritious and preferred food types for crustacean grazers, and enhance trophic transfer efficiencies in planktonic webs (Jónasdóttir et al. 1995, Müller- Navarra et al. 2000, 2004, Park et al. 2003), boosting energy transfers to larger consumers. A thorough evaluation of the potential significance of temporal phytoplankton blooms for higher trophic

Pan-American Journal of Aquatic Sciences (2007), 2 (1): 13-22

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