Transport of fresh and resuspended particulate organic material in the Baltic Sea — a model study

Abstract

A fully coupled high-resolution 3-dimensional biogeochemical–physical ocean model including an empirical wave model was used to investigate the long-term average (1970–2007) distributions and transports of resuspended matter and other types of suspended organic matter in the Baltic Sea. Modelled bottom types were compared to observations and the results showed that the model successfully managed to capture the horizontal, as well as the vertical, distribution of the different bottom types: accumulation, transport and erosion bottoms. The model also captured well the nutrient element contents in the sediments. On average the largest contribution of resuspended organic carbon to the transport of total organic carbon is found at erosion and transport bottoms. Although the relative transport of resuspended organic carbon at deeper accumulation bottoms in general is low (< 10% of total), the central parts of the sub-basins act on average as sinks that import organic matter while the more shallow areas and the coastal regions acts as sources of organic carbon in the water column. This indicates that the particulate organic matter produced in erosion and transport areas might be kept in suspension long enough to be transported and settle in less energetic areas, i.e. on accumulation bottoms.

Introduction

Organic matter produced in the photic zone of the ocean or transported to the sea from land and atmosphere may sink through the water column to the sea floor and contributes in this way to the benthic pool of nutrients. Decomposition and oxidation of this organic matter in sediments play an important role e.g. for the regulation of deep water oxygen concentrations and the internal loading of dissolved nutrients in the Baltic Sea (e.g. Conley et al., 2009). Eutrophication, which is a large environmental problem in the Baltic Sea, is reflected during recent decades in the larger and more frequent phytoplankton blooms of e.g. cyanobacteria and increased bottom areas with hypoxia (O₂ < 2 ml l⁻¹) (e.g. Conley et al., 2009, Diaz and Rosenberg, 2008, Wulff et al., 2001a). The increased nutrient concentrations in the top layer of the water column lead to high biomass production and consequently to higher sedimentation of organic matter.

When the critical shear stress (τ_c) on the sea floor is exceeded sediment particles are lifted up into the overlying water and resuspension is induced. Resuspension is a common physical process that occurs everywhere in the marine environment, in coastal areas as well as in the deep sea (Gross et al., 1988, Thomsen et al., 1994, Vangriesheim and Khripounoff, 1990). The shear stress can be a result of bottom currents induced from wind waves or from barotropic (differences in sea levels) and baroclinic (differences in density) forcing mechanisms. In contrast to many other coastal areas of the world, the tides in the Baltic Sea are negligible and are not contributing to resuspension events. Resuspension can also be induced by biological activity (Graf and Rosenberg, 1997) or by anthropogenic perturbations such as trawling and dredging. Bottom friction created by wind waves is an important element of sediment transport (e.g. Bobertz et al., 2005, Christiansen et al., 2002, Jönsson et al., 2005, Lund-Hansen et al., 1999, Schwab et al., 2006) and it is therefore essential to have a consistent wind wave model included to describe the sediment transport between the sea-floor and the water column.

Transport of resuspended organic material between different bottom types seems to be an important process in the ecological system and it has been suggested that particles settling at accumulation bottoms in the deeper parts of the Baltic Sea often originates from shallower areas (e.g. Glasby and Szefer, 1998, Jonsson et al., 1990). In the Baltic Sea the sediment at depth as great as 80 m may be affected by wave-induced resuspension at least once a year (Jönsson et al., 2005). As sediment particles are in suspension in the water column, they can move above and along the bottoms with ambient water currents and in this way strongly influence the spatial distribution of the benthic nutrient pool. Christiansen et al. (1997) investigated the transport of nitrogen and phosphorus due to resuspension in the Kattegat and found that wave induced resuspension was more important than current induced resuspension for the nutrient dynamics in near shore areas. They concluded that resuspension effects on the nutrients should be taken into consideration when nutrient budgets in shallow-water areas are studied. Transports of resuspended organic material from shallow to deeper areas were found to be of importance also in a field study in the southern Baltic Sea (Arkona Basin) (Christiansen et al., 2002).

Repeated resuspension–deposition events may move and expose organic material containing nitrogen and phosphorus to different bottom water conditions, like different oxygen concentrations. This may change rates and patterns of sediment–water exchange of the nutrients mobilised during degradation of the organic material (Almroth et al., 2009, Conley et al., 2009). The impacts of resuspension on degradation rate of organic material in the sediment and on fluxes of different solutes between sediment and overlying water have been studied by a number of research teams (e.g. Almroth, 2008, Almroth et al., 2009, Blackburn, 1997, Spagnoli and Bergamini, 1997, Ståhlberg et al., 2006, Tengberg et al., 2003, Wainright, 1987, Wainright, 1990, Wainright and Hopkinson, 1997). Some results indicated that the degradation rate of organic material was affected by resuspension while others found no effects. Also the conclusions regarding the impact of resuspension on benthic fluxes of solutes varied between the different studies. Most of these studies were performed in the laboratory or with models and in other parts of the world; only Almroth et al. (2009) performed their studies both in-situ and in the Baltic Sea. Their results from measurements using an autonomous benthic lander showed no significant effects of resuspension on benthic nutrient fluxes or on the degradation rate of organic material. Further, Christiansen et al. (1997) found that the oxygen penetration depth into the sediment decreased after resuspension indicating that oxygen consumption increased. This was verified also in the in-situ study by Almroth et al. (2009), who showed that oxygen consumption was significantly enhanced due to resuspension, which most likely was related to stimulated oxidation of dissolved reduced inorganic compounds in the sediment.

An important factor determining the spatial distributions of organic matter and nutrient elements in sediments is pattern and mode of transport of the particulate material. There is however still an incomplete understanding of the long-term transports and the final deposition areas of resuspended matter as well as suspended organic matter in the Baltic Sea. The biogeochemical importance of resuspended organic matter relative to other sources of organic matter in the deep Baltic Sea also needs to be quantified. In the present investigation we therefore used a fully coupled high-resolution 3-dimensional biogeochemical–physical ocean model to investigate the distributions and transports of resuspended matter and other types of suspended organic matter. The present 2 nautical mile (nm) model with new atmospheric forcing is based on the 6 nm model presented by Eilola et al. (2009). We also used a simplified wave model and a calibrated uniform τ_c for the entire Baltic Sea to calculate the wave and current induced shear stress and thus the resuspension of organic material.

The aim of the present study was to investigate: 1) the importance of resuspension for the transport and redistribution of particulate organic matter; 2) the distribution of resuspended organic material in the water column and sediments; and 3) a method to classify the modelled bottom types.