Lake Pontchartrain provides wintering habitat for a continentally-significant proportion of Lesser Scaup (Aythya affinis), a species of conservation concern since steep population declines began in the 1980s. Mid-winter waterfowl surveys indicate dramatic variation in scaup abundance (221–1,194,907), though the mechanisms driving these fluctuations are unknown. Previous studies have shown that scaup feed primarily on mollusks, and so changes in the benthic prey community (species composition, abundance, and size classes) have the potential to strongly influence scaup dynamics on the lake. Benthic communities are in turn shaped by both natural (e.g. hurricanes and droughts) and anthropogenic (spillway openings) forces, potentially creating a lagged bottom-up trophic cascade that ultimately affects scaup populations. To diagnose trophic linkages and variation in scaup abundance, we collected ducks (n=60) and paired benthic samples from the field to evaluate diet preferences, and then used this information to guide analysis of pre-existing long-term datasets on scaup populations and the benthic community.
The most common prey species was Rangia cuneata, accounting for 43.49% of all food items. The most commonly consumed size class of R. cuneata was between 5-9.99 mm (58.78%). Compared to 2004 (255 individuals/m2), R. cuneata (< 21 mm) practically disappeared from the lake in the winter of 2005 due to Hurricane Katrina (31 individuals/m2), but rebounded to above pre-Katrina levels (384 individuals/m2) the following year. Likewise, there were less than 500 scaup estimated on the lake in the winter of 2005, but over 800,000 birds the following winter. Droughts and spillway openings produced similar patterns of immediate decline in R. cuneata and scaup abundances (though the declines were less substantial than after hurricanes) with increased populations the following year. These disturbances appear to reset the size-class succession of benthic invertebrates; the intervening periods were characterized by an increasing abundance of larger bivalves, and accordingly fewer scaup. Scaup populations on Lake Pontchartrain appear to be closely linked to R. cuneata populations, which in turn are affected by disturbance events that alter salinity and water quality. Our results provide valuable information on trophic linkages in an estuarine system that is particularly prone to the increasingly frequent and severe disturbance events predicted with climate change.