I think there is not a single reason for a high development of Surirella in your estuary. Surirella is motile and commonly epipelic and therefore you easily find it in the estuary because there is a lot of mud. If you observe a big increase in its population, you will unlikely get to know why Surirella and not other epipelic diatom such as Navicula o Nitzschia grows. These might perfectly have grown instead. Moreover, keep in mind that you are talking about a whole genus which comprises many marine and freshwater species.

I think it is better to think in terms of algae biomass and/or groups. For example: Why do I observe a growing in planktic algae / benthic algae / cyanobakteria / green algae...? In your case, I do not know you study area and its problems and pollution sources, but I would probably ask myself why did I find an increase in benthic diatoms, for example, supposing you observed it. If you have just seen a switch from Navicula to Surirella, for example, it may have been just part of the annual sucession in your ecosystem and may not have any easy-to-find reason (and the actual reason may not be important at all in terms of monitoring or assesment).

Thank you. Gonzalo Martín for answering my question.

N:P ratio is the key factor