Odontella spec.

I took a water sample with a plankton net from the tidal flat side of the German barrier island Spiekeroog on a stormy day in November 2012. And i found this diatom in there. This photo was edited with the programme Picolay [link] and consists of about 30 stacked images.
The diatom is either an Odontella regia (Simonsen, 1974) or an Odontella mobiliensis (Grunow 1884).

Odontella regia
The diatom Odontella regia is very common in temperate seas. In girdle view valves appear rectangular. Short hornlike processes with blunt rounded tips arise from the corners of the valves. Cells are often found united into chains by their labiate processes. These processes have split
tips which can be hooked. Healthy cells, like the one in the photo, contain numerous platelike chloroplasts.
Lenghts of the large diameter of the apical axis can range from 90 - 200 µm and from 45 - 100 µm for the small diameter of thr transapical axis.

Odontella mobiliensis
Odontella mobiliensis is a cosmopolitan species which occurs in plankton communities throughout the whole year. As shown on the photo, cells are cushion-shaped and roughly rectangular in girdle view. The corners of the valves are
drawn out into short processes with blunt rounded tips. Between these processes the cells have two long spines at each side which they use to connect themselves to form
chains. Cells contain a numerous amount of small, platelike chloroplasts.
Lenghts of the large diameter of the apical axis can range from 40 - 200 µm and from 18 - 90 µm for the small diameter of thr transapical axis.

As you can see from the short descriptions, both species are very similar and I'm not an expert on phytoplankton so I can't tell which species exactly I found, but it has to be one of those two.