  Patagonian fjords' area is one of the largest estuarine regions in the world.
Every one of its water bodies displays a unique hydrodynamic behavior with
enormous effects on the biogeochemical characteristics of the ecosystems. In
this context, algal blooms are ecological phenomena of major relevance.
Numerical simulation has proved to be a promising tool to understand their
impacts. It has not been used for studying algal blooms in this zone. This
article focuses on proposing a novel numerical model for simulating brief algal
blooms occurring in water bodies of western Patagonia. The proposed model
presents a trade-off between complexity and applicability since field-data
sparsity in the zone discourages using more sophisticated approaches. The model
is based on a two-layer description of the water column. The first layer
represents the euphotic zone where an embedded biogeochemical model of
NPZD-type is used to model a mass-conserving trophic web. High intensity wind
drives the water column mixing, introducing an upward flux of nutrients that
boosts high rates of primary production. A time-dependent Gaussian pulse is
used to describe this process. Mass losses due to detritus sinking are also
included. Then, the ecosystem's dynamics is represented by means of an
externally forced, non-autonomous system of ordinary differential equations
which is characterized by strictly positive trajectories but that it is not
longer mass-conserving. A structure-preserving time integrator based on a
splitting-composition technique is designed for solving the system's equations.
It is cast as a three-steps algorithm and provides an exact estimations of
biomass fluxes. Additionally, a genetic algorithm-based tool is used to
calibrate the model's parameters in realistic scenarios. The proposed model is
applied im a detailed study of a winter bloom in an austral fjord.
