Diving behaviour in a Lesser Black-Backed Gull

While walking with Zoe around the Reading University lakes this morning, I noticed a lesser black-backed gull (Larus fuscus) sitting on the water.  Suddenly, it flapped its wings as if taking off but rose only a few feet into the air and then dived head-first back into the water, almost immediately coming to the surface again.  I saw it perform this trick twice, so it was obviously deliberate.  Maybe it was trying to catch a fish and needed to get a certain amount of momentum to get deep enough to reach it.  After the second dive, the gull then slowly meandered its way around the lake as if looking for something else to dive for, but we didn't see it dive again.