copepods and fish larvae (Lasker 1975, Mullin & Brooks 1976, Ware et al. 1981, Mullin et al. 1985, MacKenzie et al. 1990, Mullin 1993). Zooplankton may escape starvation by exploiting zones of high food concentration like fronts or subsurface layers of high chlorophyll concentration (Subsurface Chlorophyll Maxima, SCM). Laboratory studies showed that a variety of zooplankters can effectively locate and utilize thin layers of elevated food concentration, leading to higher ingestion and secondary production rates (Tiselius 1992, Saiz et al. 1993, Ignoffo et al. 2005). Thus, processes affecting phytoplankton biomass levels and the formation and persistence of food-rich layers are presumed particularly important for energy transfers in pelagic food webs.
The presence of SCM can result from physical (formation of a sharp pycnocline, light penetration, nutrient diffusion across the pycnocline, vertical shear at fronts), biological (functional response to irradiance, nutrient absorption kinetics, community succession) and behavioural processes (vertical migration) (Cullen & Eppley 1981, Iriarte & Bernal 1990, Castro et al. 1991, Bjørnsen & Nielsen 1991, Franks 1992, Djurfeldt 1994, Franks & Walstad 1997).
The vertical distribution of microplankton is also greatly affected by wind mixing of surface layers associated to storm events (Haury et al. 1990). These events may erode SCM and affect the feeding environment and survival of larval fish (Lasker 1975). However, in situ and experimental evidence also suggest that turbulent mixing may favour pelagic productivity; the alternation of vertical mixing and stratification of the water column leads to surface water fertilization and enhanced primary production (mainly by diatoms), which can be transferred up the food chain to heterotrophic consumers (Kiørboe 1993, Mann 1993, Mann & Lazier 2006).
Material and Methods Study site
Coliumo Bay is a small embayment in Central-South Chile (36º32’ S, 72º57’ W, Fig.1) that opens to the Pacific Ocean with its main axis (ca. 3 km) oriented North-South. Coliumo River flows into the head of the bay and dilutes marine waters to some extent (Llancamil 1982). Water depth is < 5 m along its perimeter up to 400 m from the shoreline, reaching a maximum of ca. 20 m near the mouth. The organic matter content of the sediment is positively correlated with depth and about 20% of the bottom has > 10% organic matter (Soto 1997, Rivas 1997). Water is moderately transparent (light attenuation coefficient Kd ca. 0.339; Calliari & Antezana 2001) and between 3 and 4% of incident radiation reaches 10 m depth. Predominant winds associated to the South Pacific Anticyclonic gyre blow from S-SW during periods of fair weather (Saavedra & Foppiano 1992), but high topography adjacent to the bay (chilean coastal mountain range) largely protects it from such winds. However, strong winds from the N usually occur during wintertime associated to heavy rains (Saavedra & Fopiano op. cit.); under such conditions the bay is fully exposed to the wind and to the swell from open ocean.
As part of a broader study at Coliumo Bay (South-Central Chile), a field experiment was designed to assess the short-term variability in the biomass and vertical distribution of the phytoplankton in response to wind forcing. By the end of the sampling period a storm hit the study site enabling to explore the effect of extreme conditions on the microplankton assemblage (denoting here all organisms in the size range of 5 – 200 µm, see Methods) beyond the normal wind regime. Based on observations before, during and after that storm event I here address the question of how overall phytoplankton biomass (as chlorophyll-a) and its vertical distribution, and the microplankton assemblage structure responded to a storm event at a relatively shallow coastal ecosystem.
Figure 1. Map of Coliumo Bay (A) in a regional (B) and general scale (C). In A, black dot inside the bay indicates the position of the sampling station; EBM stands for Marine Biological Station (UdeC), where the wind sensors were placed.