Kerteminde Fjord
The fjord-system consisting of Kerteminde Fjord and Kertinge Nor (Fig. 1) covers an area of 8.5 km2 and has a mean water depth of approximately 2 m and a maximum depth of 8 m. The fjord has a sill at its mouth to the open sea (Great Belt). The discharge over the sill is forced by a diurnal tide with an average amplitude of approximately 20 cm. The tide gives rise to maximum discharges at the fjord entrance of 100-200 m3 s-1. The fresh water input of 0-0.05 m3 s-1 is negligible with respect to the water exchange of the fjord-system. The salinity in the central part of the system varies typically between 14 and 22 ‰ over the year. The temperature ranges between 0 and 22 Celcius degrees.
Fig. 1. Kerteminde Fjord/Kertinge Nor.
Water exchange of the fjord-system is governed by density driven circulation. The salinity in the Great Belt outside the fjord varies as a result of changing flow situations. Outflow of water from the Baltic Sea gives salinities down to 10 ‰ whereas inflow to the Baltic Sea gives salinities up to 27 ‰ in the upper layer of the Great Belt. Because saline water is more dense than fresh water the salinity variations cause longitudinal density variations from the inner part of the fjord-system to the mouth. As a consequence of longitudinal density gradient, density driven vertical circulation occurs. When dense water by tidal forcing is flushed over the sill it will flow down below the fjord water and give rise to a density driven circulation system within the entire fjord-system. When, on the other hand, light water is forced into the fjord the circulation is in the opposite direction. On an annual time scale the two circulation directions have equal probability. A qualitative illustration of the strength and the direction of the exchange of water is given in Fig. 2.
Fig. 2. Kerteminde Fjord/Kertinge Nor. Density driven water circulation.
Kertinge Nor
The biological structure of the shallow cove of Kertinge Nor in the inner part of Kerteminde Fjord is shown in Fig. 3. The water column is often extremely clear which allow sufficient light penetration to the bottom where a significant benthic primary production of filamentous algae and eelgrass may be found. The dense algal mat is important for the control of the nutrient flux from the sediment into the water column. Below the algal mat the sediment is black and sulphidic due to anoxic conditions and without living animals.
Fig. 3. Biological structure in Kertinge Nor during summertime. Three food-chains may be identified:
1) phytoplankton → ascidians (Ciona intestinalis)
2) epiphytic diatoms → epibenthic harpacticoids → jellyfish (Aurelia aurita) + sticklebacks
3) macrophytes → detritus → decomposing microorganisms.
The water processing capacity of the jellyfish population is very high, with a maximum rate obtained in early September, where the jellyfish population daily may process a water volume corresponding to approximately 13 times the whole water volume of Kertinge Nor. This shows that Arelia aurita can control zooplankton in the cove during summer and fall.
Laboratory experiments have proved that the medusae are food limited at in situ zooplankton concentrations found during daytime. However, the density of harpacticoids in the water column during night can exceed the density during day by a factor of 20 and night-swimming harpacticoids may therefore be an important food source for the jellyfish in Kertinge Nor.
The filter feeding Ciona intestinalis (Fig. 4) may exert a high grazing pressure on phytoplankton which partly explained the low observed phytoplankton biomass. In particular, during late summer and fall, theCiona population can reach densities of approximately 250 individuals per m2. During fall, the dense population of C. intestinalis has the potential capacity to filter the total water volume of Kertin ge Nor 0.2 to 1.2 times daily, and the mean residence time of an algal cell in the water column (t½) may only be about 7 hours in September.
Fig. 4. Filter-feeding ascidians (Ciona intestinalis) in Kertinge Nor.
The Great Belt
The Great Belt is part of the 3 Danish straits (the Great Belt, the Little Belt and the Sound) connecting the brackish Baltic Sea with the saline North Sea. The water depth in the straits is shallow with typical depths between 10 and 20 m. In the Great belt the boundary toward the Baltic Sea is given by the Darss sill at a depth of approximately 17 m. Towards the North the boundary between the Kattegat and the Skagerak is given by the sloping bottom toward the deep Norwegian Trench (Fig. 5). 
Fig. 5. Bathymetry of Danish waters. The Danish straits constitute a transition zone between the brackish Baltic Sea and the saline North Sea. The Great Belt is the middle of the 3 straits.
With respect to hydrography, a distinction is made between the net and the instantaneous flow condition in the Great Belt. The net current is northbound due to the surplus of freshwater flow into the Baltic. Due to the general estuarine circulation caused by the density gradient from the Baltic to the North Sea the mean current in the upper layer is directed out of the Baltic whereas the mean current in the bottom layer is directed towards the Baltic (Fig. 6). The instantaneous flow condition, though, is governed by the actual water level difference between the Kattegat and the western Baltic as well as the estuarine circulation. The dominating forcing is here the combined effect of meteorological forcing (wind and solar heating) and tidal forcing. Hence, with irregular intervals, on a time-scale of days, the flow through the Great Belt is seen to oscillate between south- and northbound with peak discharges being 10 to 15 times the mean discharge. The variable density in the Great Belt associated with this dynamic hydrographical system assists in maintaining the density driven circulation and water exchange in e.g. the Kerteminde fjord-system and the Odense Fjord with openings to the Great Belt.
Fig. 6. North-South transect through Danish waters, that are characterized by large horizontal salinity differences (psu = from 8 to 35). Less salty and thus lighter brackish water flows northward in the surface while a more salty layer near the bottom flows south-ward.
Odense Fjord
The total area of Odense Fjord is 60 km2, the outer fjord is about 50 km2 with a mean depth of 2.7 m and the inner fjord is about 10 km2 with a mean depth of 0.8 m. The catchments area to the fjord is, large, approximately 1,000 km2 (about one third of the island of Fyn) most of which is agricultural land; the freshwater enters the fjord mainly via Odense River (Fig. 7). In 2000, Odense Fjord received 2,300 tonnes of nitrogen and 54 tonnes of phosphorus. Due to this high loading, Odense Fjord is eutrophicated.
Fig. 7. Odense Fjord viewed from Odense facing northeast. Photo: J. Kofoed Winther.
Odense Fjord is characterized by a large biomass of filter-feeding polychaetes (Nereis diversicolor), clams (Mya arenaria) and cockles (Cerastoderma glaucum), which together make up about 70 % of the total animal biomass. Other species of bivalves in Odense Fjord are Mytilus edulis, Macoma balthica, Scrobicularia plana, and Ensissp.
Compared to other shallow marine waters, the density of the facultatively filter-feeding Nereis diversicolor is unusually high in the inner part of Odense Fjord, and therefore it has been assumed that N. diversicolor along with Mya arenaria and Cerastoderma glaucum (Fig. 8) play an essential role for the regulation of the biomass of phytoplankton in the inner part of the fjord. The grazing impact of the filter-feeding zoobenthos in Odense Fjord, with focus on the inner part, has recently been studied.
Fig. 8. Mya arenaria
The distribution of Nereis diversicolor, Mya arenaria andCerastoderma glaucum in the shallow Odense Fjord were mapped in 2000 and the data showed that these three dominating species of benthic filter-feeding macro-invertebrates can filter a volume equivalent to the total volume of water in the inner part of the fjord in Q = 0.29, 0.46 and 4 d, respectively, and that the potential grazing impact, expressed as mean residence time of phytoplankton under well mixed conditions, is t½ = 0.20, 0.32 and 2.8 d, respectively.
The total potential grazing impact exerted by the three species is Q= 0.17 d and t½ = 0.12 d or less than 3 h. This indicates that especially N. diversicolor and M. arenaria may exert a pronounced controlling impact on the phytoplankton in the inner part of Odense Fjord
However, it must be emphasised the estimated grazing impacts are potentials that may only be realised if a decisive prerequisite is fulfilled, namely that the filter-feeding animals on the bottom are exposed to the whole water column by effective vertical mixing of the water. In the shallow inner part of Odense Fjord with a mean depth of only 0.8 m the water may often be well mixed by wind action. More systematic studies with focus on this problem are now in progress.
Surplus of nutrients has increased the biomass of macroalgae, such as sea lettuce Ulva lactuca and horsehair seaweed Chaetomorpha linum which appear in large quantities in the fjord during the summer (Fig. 9). In the 1980's, U. lactuca appeared in the inner part of the fjord during summer with extremely high biomasses (1 kg dry mass per m2) and with an annual production of about 1000 tons of carbon, or twice the annual phytoplankton production in the same area. Since then especially the phosphorus and the summer-nitrogen loads have decreased, coincidently with a decrease in the abundance of macroalgae in the fjord.
Fig. 9. Mass occurrence of sea lettuce (Ulva lactuca) in the inner part of Odense Fjord.
PUBLICATION FROM 2008 ABOUT ODENSE FJORD, KERTINGE NOR AND KERTEMINDE FJORD (PDF)
Article in Danish about Kertinge Nor (Artikel fra Kaskelot nr. 114, 1997)