Richard Hofmeister

Wind and tidal mixing controls on stratification and dense water outflows in a large hypersaline bay

In Shark Bay, a large inverse estuary in Western Australia, longitudinal density gradients establish a gravitational circulation that is important for Bay-ocean exchange and transport of biological material such as larvae. The relative contributions of energy from wind and tidal mixing on the control of vertical stratification and gravitational circulation were investigated using the three-dimensional baroclinic ocean circulation model (General Estuarine Transport Model, GETM). In this large inverse estuary, the effects of the winds and tides on vertical mixing were found to be of similar magnitude. A critical depth of ∼15 m was identified that determined whether winds or tides or a combination of the two was required to create vertically mixed conditions. Where it was shallower than the critical depth, either the wind or tide could fully mix the water column. In contrast, a combination of both winds and tides was required to mix the deeper channels. Density-driven circulation peaked 0–3 days after periods of maximum stratification, resulting in a fortnightly modulation of dense water outflows along the seabed following the tidal stage. Salt flux calculations provided new evidence for the predominance of outflow through the deeper northern entrance channel where outflows persisted through all stages of the tide. In contrast, outflows through the western channel were more intermittent with a stronger tidal component. Wind driven lateral circulation between the entrances was also important and could temporarily reverse the circulation during northerly wind events.

Estuary-type circulation as a factor sustaining horizontal nutrient gradients in freshwater-influenced coastal systems

Estuary-type circulation is a residual circulation in coastal systems with horizontal density gradients. It drives the accumulation of suspended particulate matter in coastal embayments where density gradients are sustained by some freshwater inflow from rivers. Ebenhöh et al. (Ecol Model 174(3):241–252, 2004) found that shallow water depth can explain nutrient gradients becoming established towards the coast even in the absence of river inflow. The present study follows their concept and investigates the characteristic transport of organic matter towards the coast based on idealised scenarios whereby an estuary-type circulation is maintained by surface freshwater fluxes and pronounced shoaling towards the coast. A coupled hydrodynamical and biogeochemical model is used to simulate the dynamics of nutrient gradients and to derive budgets of organic matter flux for a coastal transect. Horizontal nutrient gradients are considered only in terms of tidal asymmetries of suspended matter transport. The results show that the accumulation of organic matter near the coast is not only highly sensitive to variations in the sinking velocity of suspended matter but is also noticeably enhanced by an increase in precipitation. This scenario is comparable with North Sea conditions. By contrast, horizontal nutrient gradients would be reversed in the case of evaporation-dominated inverse estuaries (cf. reverse gradients of nutrient and organic matter concentrations). Credible coastal nutrient budget calculations are required for resolving trends in eutrophication. For tidal systems, the present results suggest that these calculations require an explicit consideration of freshwater flux and asymmetries in tidal mixing. In the present case, the nutrient budget for the vertically mixed zone also indicates carbon pumping from the shelf sea towards the coast from as far offshore as 25xa0km.

Modeling the Role of pH on Baltic Sea Cyanobacteria.

We simulate pH-dependent growth of cyanobacteria with an ecosystem model for the central Baltic Sea. Four model components—a life cycle model of cyanobacteria, a biogeochemical model, a carbonate chemistry model and a water column model—are coupled via the framework for aquatic biogeochemical models. The coupled model is forced by the output of a regional climate model, based on the A1B emission scenario. With this coupled model, we perform simulations for the period 1968–2098. Our simulation experiments suggest that in the future, cyanobacteria growth is hardly affected by the projected pH decrease. However, in the simulation phase prior to 1980, cyanobacteria growth and N2-fixation are limited by the relatively high pH. The observed absence of cyanobacteria before the 1960s may thus be explained not only by lower eutrophication levels, but also by a higher alkalinity.

Modular System for Shelves and Coasts (MOSSCO v1.0) – a flexible and multi-component framework for coupled coastal ocean ecosystem modelling

Abstract. Shelf and coastal sea processes extend from the atmospherenthrough the water column and into the seabed. These processes reflectnintimate interactions between physical, chemical, and biological states onnmultiple scales. As a consequence, coastal system modelling requires a highnand flexible degree of process and domain integration; this has so far hardlynbeen achieved by current model systems. The lack of modularity andnflexibility in integrated models hinders the exchange of data and modelncomponents and has historically imposed the supremacy of specific physical drivernmodels. We present the Modular System for Shelves and Coasts (MOSSCO;n http://www.mossco.de ), a novel domain and process coupling systemntailored but not limited to the coupling challenges of and applicationsnin the coastal ocean. MOSSCO builds on the Earth System Modeling Frameworkn(ESMF) and on the Framework for Aquatic Biogeochemical Models (FABM). It goesnbeyond existing technologies by creating a unique level of modularity in bothndomain and process coupling, including a clear separation of component andnbasic model interfaces, flexible scheduling of several tens of models, andnfacilitation of iterative development at the lab and the station and on thencoastal ocean scale. MOSSCO is rich in metadata and its concepts arenalso applicable outside the coastal domain. For coastal modelling, itncontains dozens of example coupling configurations and tested set-ups forncoupled applications. Thus, MOSSCO addresses the technology needs of angrowing marine coastal Earth system community that encompasses very differentndisciplines, numerical tools, and research questions.