✹ About Wednesday 4 September 1661 Nate Lockwood on 5 Sep 2014 • Link Oysters, mussels, etc. are filter feeders so if there is a bloom of dinoflagellates (algae) those shellfish could readily have accumulated toxins. Blooms tend to occur in warm weather, shallow calm seas and estuaries after rains and are due to the presence of nutrients such as nitrates and potassium in the water column which can come from fertilizer application but are also present in sewage. Manure was a primary fertilizer in the 16 and 17 centuries so was probably used in Pepys' time, too. When in a military survival course in June many years ago we were forbidden to collect shellfish as there was an algal bloom so our pot was a bit empty.