Lesser Black-backed Gulls (Larus fuscus) foraging on estuarine macrozoobenthic prey?

Gulls, in particular Lesser Black-backed gulls, are opportunistic omnivores, but, as visible here in Portugal, their main diet is either marine based (in particular fish) or whatever is available to them at the landfills. Throughout the winter on a daily basis I could observe that, at least around Aveiro, a few thousand gulls (primarily Lesser Black-backed -, fewer Black-headed - and some Yellow-legged Gulls) were foraging on the mudflats and inactive saltpans.

By now, most of the gulls have migrated back to their breeding grounds further to the north, but there are still a few hundred actively searching for food. I suspect that a majority of the gulls here has a strong relationship with the landfill; at least most of the rings that were read here were also seen at Taboeira.

The birds show a preference for areas that were covered with a layer of algae and it seems that the prey that they take occurs either in or directly under this layer.

I’ve seen them share the same foraging areas with Black-tailed Godwits (Limosa limosa) which at first sight appeared to be after the same type of prey that lives in or under the algae. It remains unknown to me what exactly these gulls are foraging on; maybe one of the information panels at the saltpans provides us with the answer?