Blame it on the weather

The diatom Pseudo-nitzschia is a source of the neurotoxin domoic acid (DA), and Pseudo-nitzschia blooms in Monterey Bay, California have been linked to the deaths of many marine mammals and birds. It is well documented that DA enters the food chain when pelagic and benthic organisms feed on toxin-producing phytoplankton. This paper focuses on strandings of California sea lions (Zalophus californianus) exhibiting acute DA intoxication. In the last few years, concentrations of Pseudo-nitzschia and/or DA, and strandings of animals have not always been directly related. The authors suggest that a more complex pattern of events is to blame, and investigate other causative factors implicated in the occurrence of the strandings.

Data was collected over a 10 year time period, from 1998 to 2007. The authors investigated temporal patterns of two toxin-producing species of Pseudo-nitzschia, P. australis and P. multiseries, taking water samples from two locations, one near-shore and the other around 20km off-shore. Species-specific molecular probe methods were used to determine the abundance of the Pseudo-nitzschia species. A high performance liquid chromatography (HPLC) method was used to measure DA concentrations in particulate material (i.e. cells) for samples taken during 1999-2005. Chlorophyll a concentrations were measured at the near-shore site, and used as a biomass indicator of phytoplankton.

Seasonal patterns of Pseudo-nitzschia species were observed, with higher cell numbers in the spring. Toxic Pseudo-nitzschia species were most abundant when levels of chlorophyll a were low or moderate. There was strong correlation between abundance of the toxic species and DA concentrations.

Although the number of sea lions stranding with DA poisoning varied each year, acute stranding events took place in 1998, 2000 and 2007. There was significant correlation between numbers of sea lions showing DA intoxication and the abundance of toxic species and concentrations of DA. However, high levels of toxic species and DA which were observed in 2002, 2003 and 2004 did not result in DA related strandings. It is suggested that the acute stranding events were the result of a combination of factors. The sea lions have seasonal patterns of breeding and foraging movements. Weather events such as El Niño/Southern Oscillation (ENSO) led to changes in the movement patterns of the sea lions. ENSO events decrease upwelling and therefore nutrient availability in surface waters, and differentially affect productivity along the coast line. This causes shifts in abundance of prey (i.e. sardines and anchovies) for sea lions and forces them to forage elsewhere. Sea lions moving northward from breeding sites to find prey were likely exposed to toxic Pseudo-nitzschia blooms occurring in the bay at the time, as the ENSO events cause shifts in phytoplankton communities, favouring the normally less abundant dinoflagellates and diatoms.

I found this paper to be a bit confusing at times. The discussion is quite lengthy and needs to be better structured to aid the reader’s understanding. It does however highlight the fact that incidences of strandings are a result of a complex set of interactions, and that DA concentrations alone cannot be used to predict stranding events along the coast.

A review of: Bargu, S., Silver, M., Goldstein, T., Roberts, K. and Gulland F. (2010) Complexity of domoic acid-related sea lion strandings in Monterey Bay, California: foraging patterns, climate events, and toxic blooms, Mar Ecol Prog Ser, 418: 213-222.