Intra-annual variation in Hg threatens waterbird reproduction

Many species of wildlife are at risk to mercury bioaccumulation. Wildlife dependent on aquatic ecosystems are at a particularly high risk because mercury often ends up in runoff entering streams and lakes and  because conditions in aquatic environments enhance production of Methylmercury (MeHg); the most toxic form of Hg for wildlife. This means that managers often pay careful attention to monitoring Hg in aquatic environments - but how do they know the best time of year to monitor?

Ariel photo looking East into San Francisco Bay, California from the Pacific Ocean

A recent study by Collin Eagles-Smith and Joshua Ackerman (2010) looked at temporal variation in Total Hg (THg) in two fish species (Threespined stickleback and Longjaw mudsuckers) within the San Francisco Bay (SFB) estuary - a large and highly productive wetland surrounded by heavy urban development and prone to Hg pollution. The goal of the study was analyze temporal variation in fish Hg and to overlay that  with the reproductive stages of Forster's terns - a waterbird  that regularly consumes sticklebacks and mudsuckers within the SFB estuary.  The authors were able to use THg as an indicator of MeHg because upwards of 90% of Hg in wildlife can be MeHg and so measuring THg generally provides a good indication of MeHg.

The authors postulated that THg concentrations in fish would increase through summer as ambient environmental conditions improve for MeHg production (i.e. increased water temperature) and that it could coincide with reproduction in forster's terns. Since reproduction in waterbirds is said to be the most important endpoint of Hg toxicity in waterbirds, and because egg formation is a major excretory pathway for Hg in females it becomes important to know when they are at highest risk to MeHg production.

The results of their study found high variability in fish THg during the tern reproduction period - increasing by as much as 40% over April and May, and then declining 39% through June and July. Interestingly, Forster's terns initiate their clutch around May 20, the peak week of MeHg production in this study. But is this relevant to breeding Forster's terns?

As it turns out, the answer is yes. A previous study in 1986 by L.B. Astheimer found that egg whites (albumen) is where almost all of the Hg within eggs is stored, and that  its synthesized using nutrients consumed by the parent female over the 7 day period prior to egg laying. So in Eagles-Smith and Ackerman's study, Forster tern eggs were formed using fish-derived nutrients when those fish were experienced peak Hg concentrations during the course of the summer.

This does not bode well for Forster's terns and many other species that breed during the same time period. Reproduction is a critical period for all bird species and some have the added threat of peak contaminant concentrations in the environment.

Managers of aquatic ecosystems will do well if they incorporate temporal variation into their often (at best) annual monitoring studies of contaminants in the environment. This study highlights the importance of intra-annual variation so at worst, managers should be consistent when the sample, and at best they should sample for contaminants several times per year.