Mogens Rene Flindt

Nutrient cycling and plant dynamics in estuaries: A brief review

Eutrophication of European estuaries due to massive nutrient loading from urban areas and diffuse runoff from extensively cultivated land areas is analysed. Consequences for the ecology of estuaries, namely changes in plant species composition, which also affects heterotrophic organisms, are approached based on examples showing that the result is often a fundamental structural change of the ecosystem, from a grazing and/or nutrient controlled stable systems to unstable detritus/mineralisation systems, where the turnover of oxygen and nutrients is much more dynamic and oscillations between aerobic and anaerobic states frequently occur. Several relevant aspects are examined, namely the influence of rooted macrophytes on nutrient dynamics, by comparing bare bottom sediments with eelgrass covered sediments, primary production and the development of organic detritus, and hydrodynamics and its relations to the spatial distribution of macrophytes in estuarine systems.

Description of the three shallow estuaries: Mondego River (Portugal), Roskilde Fjord (Denmark) and the Lagoon of Venice (Italy)

The paper describes three European estuaries which were compared with respect to the dynamics between autotrophic components under the MUST-project: The Mondego River (M) (Portugal), Roskilde Fjord (R) (Denmark) and Venice Lagoon (V) (Italy). The areas of the three estuaries are (M) 3.4, (R) 125 and (V) 540 km2 and their maximum tidal ranges are (M) 3.3, (R) 0.2 and (V) 2.2 m. They are all eutrophic with high loadings of nitrogen (M) 126 t N/yr, (R) 2500 t N/yr and (V) 7000 t N/yr and high loadings of phosphorus (M) 1 t P/yr, (R) 180 t P/yr and (V) 1000 t P/yr. The dominating phytoplankton species are (M) diatoms and dinoflagellates, (R) Skeletonema sp. and (V) Amphora sp. and Chaetocerus sp. and the dominating macrophytes are (M) Enteromorpha sp., Gracilaria sp. and Zostera noltii, (R) Zostera marina and Ulva lactuca and (V) Ulva rigida and Zostera noltii. All three estuaries are frequently exposed to collapses caused by severe oxygen depletion.

Loss, growth and transport dynamics of Chaetomorpha aerea and Ulva rigida in the Lagoon of Venice during an early summer field campaign

The growth, the losses by grazing and sporulation and the advective transport of benthic macrophytes, dissolved and particulate nutrients were studied in the Lagoon of Venice during May–June 1995. The growth rate of Ulva rigida was 0.043 d−1 and the grazing rate found to be 0.01 d−1. No sporulation was observed in the study period. The advective transport of macrophytes was measured by catches in vertically exposed nets during several tidal excursions and varied from 0 to 11 kg Chaetomorpha aerea, 10 kg Zostera sp. and 9 kg Ulva sp. wwt. hour−1 per 2 m net length normalized to effective length perpendicular to the current direction. The transport of Chaetomorpha and Ulva was linearily correlated to the current velocity (r2 = 0.80 and 0.88 respectively). Vertically separated nets revealed that 89% of the Zostera sp. was transported in the top 30 cm of the water column and 65% of the Chaetomorpha sp. was transported in the deepest 30 cm of the column and the transport of Ulva sp. was dispersed over the 1 m water column. Automatically taken water samples were analysed for dissolved inorganic nutrients (carbon, nitrogen and phosphorus) and particulate nutrients during the campaign periods together with nutrients bound in macroalgae and seagrasses. The advective transport of dissolved and non-macrophyte bound, particulate nutrients were calculated by a 2-dimensional, hydrodynamic model. Less than 1% of the nutrients were transported as dissolved inorganic nitrogen and phosphate, 3–4% was transported as non-macrophyte, particulate nitrogen and phosphorus and more than 90% of the nutrients were transported as macrophyte bound nutrients. It is therefore obvious that mass balance calculations for shallow estuarine systems should include advective transport of dead and alive macrophytes.

Modelling advective transport of Ulva lactuca (L) in the sheltered bay, Møllekrogen, Roskilde Fjord, Denmark

Biomass, grazing, sporulation, and growth of the free-floating macroalgae Ulva lactuca were measured in a small bay of 420 000 m2 inside Roskilde Fjord, Denmark. The measurements where made at 10 stations with a spatial resolution of a few hundred meters, and a temporal resolution of two times per week. A two-dimensional transport-dispersion model of Ulva lactuca was set up for the area. Erosion of the macroalgae was modelled as a function of shear stress produced by horizontal- and orbital current velocity. The measured biomass abundance, grazing rates, sporulation rates and growth rates were used as input to the model, and two scenarios were simulated: one with and one without simulated transport-dispersion. Measured biomass were applied as initial values for each consecutive simulation period of 3–4 days. Simulation results exhibited significantly better agreement with measurements in the transport-dispersion scenario. This result strongly supports the working hypothesis, that advective transport is significant in controlling Ulva lactuca biomass, and that advective transport, therefore, must be included in a general model of Ulva lactuca dynamics.

Significance of advective transport of Ulva lactuca for a biomass budget on a shallow water location

Ulva lactuca biomass, growth and grazing rates and drifting biomass was measured in the early summer period at a 1 m deep near-shore station in a sheltered bay in Roskilde Fjord, Zealand, Denmark. The expected biomass was calculated for each sampling date and compared with the measured. Regression analyses were performed between drifting biomass and the relative difference between measured and calculated biomass (R2 = 0.65, P < 0.054) and between wind velocities and drifting biomass (R2 = 0.42, P < 0.057). Although not significant at the 5% level, the results indicated that advective transport was significant for balancing the biomass budget of U. lactuca. It is therefore proposed that dynamic modelling of U. lactuca should be based on a transport/dispersion model.

Modelling of an estuarine eutrophication gradient

Roskilde Fjord represents an estuarine eutrophication gradient with annual average salinities varying from 11 to 18°C and average summer chlorophyll concentrations ranging from 3 to 25 mg m−3. A PC based one-layer and one-dimensional hydrodynamic model and a transport/dispersion model served as the basis for a eutrophication process model with 12 state variables. Water levels and salinities were well predicted. Objective function yielded values of 0.13 for dissolved oxygen, 3.63 for chlorophyll, 8.44 for inorganic nitrogen and 3.68 for phosphate for the water quality model during a design period covering the spring 1985. Comparisons with mass balances calculated by means of a simple box model indicated the necessity of including a resuspension and sediment transport description.

The faunal role in the degradation of the common intertidal salt marsh plant Scirpus maritimus

The aim of this work was to evaluate the role of different environmental conditions (oxic and anoxic), and the presence of macrofauna and/or meiofauna during the different steps of Scirpus maritimus L. decomposition/mineralization under controlled laboratory conditions. The results showed no significant differences between the anaerobic and the aerobic degradation of plant material, under the presence of bacteria or meiofauna. Nevertheless, under anoxic conditions sediment mineralization was enhanced, with an increase concentration of phosphorus and ammonium in the water phase. Concerning the presence of fauna, results show that, although bacterial activity was responsible for 70% of the S. maritimus leaves degradation, the presence of macrofauna together with meiofauna enhanced the leaves mineralization up to 90%. Moreover, the presence of macrofauna together with meiofauna significantly affected the decomposition of phosphorus and of nitrogen, as well as the leaves lesser labile structural parts, by increasing the mineralization of plant carbon, and raised the nutrient turnover within the system.The present study reinforces the functional link between fauna levels on the nutrient dynamics in salt marshes ecosystems, namely at the vegetation detritus/water column interface.