How to get rid of mosquitoes in a small lake
There are a number of environmentally destructive methods that would be effective, including draining the lake, covering the surface with a continuous layer of oil, or adding toxins to the water, but I'm assuming you're looking for a method that will have the minimum possible off-target effect.

Different mosquito species breed in different habitats and are thus susceptible to different control strategies, so good photos of some adults might help, but I'm assuming here that you've correctly identified the lake as the breeding site. If there are any other possible breeding sites in the area please add them to your question.

If the assumptions above are correct then, broadly speaking, your best options are habitat modification or biological control. I'm not going to go into chemical control here since as stated above I assume you want low environmental and financial cost; if this is incorrect let me know.


Habitat modification: regularly removing
plant growth within the lake and trimming vegetation overhanging the edges will limit the ability of mosquito larvae to escape predators and may have some effect. However, the recent management you describe has
probably gone some way towards this (did you
notice whether the mosquito problem got any better after the plants
were removed?)
Biological control: introduce predators or parasites into the environment. Plenty of species eat adult mosquitoes but there aren't really any that primarily feed on them (unless they're the only food source available) and many adult mosquitoes will have laid eggs already - the most effective strategy will be to target the immature population. Larvivorous fish have been shown to be effective in many parts of the world, for example fish in the genus Gambusia ('mosquito fish'). I don't know of any that have been shown to be effective and can thrive as far north as Scandinavia, but if you stock the lake with fish (combined with managing vegetation as above) I'd be surprised if you don't see some improvement.
Biological control, part 2: Use Bti (Bacillus thuringiensis var. israelensis). This is a bacterium that produces toxins which are highly specific to dipterans and don't really affect anything else. You can buy the bacterium in various formulations including sprays, tablets etc. I'm mainly familar with its use for controlling container-breeding mosquitoes, but this paper describes its use in large lakes so should give you some idea of dose required, optimal application method and time to reapplication. 


Without knowing more about your budget, available time/manpower etc I can't really help you choose between these but I suspect #3 is your best bet.