Interannual variability of shellfish toxicity in the Gulf of Maine: Time and space patterns and links to environmental variability

Six metrics characterize annual aspects of the magnitude and timing of shellfish toxicity resulting from dinoflagellates of the genus Alexandrium at >100 monitoring stations over 21 years (1985–2005) along the coast of Maine, northeastern USA. Climatologies at each station show distinct geographic patterns, generally consistent with previous reports based fewer stations/years. Earliest initiation and dates of maxima are at far western coastal stations, latest in far eastern stations. Toxicity magnitudes are highest at exposed western coastal stations and at far eastern stations inside a bay with minima in the center of the study region. Multivariate statistics group years according to similarity in station toxicity magnitude and timing. Results (a) confirm that a year of extreme toxicity (2005) differs from recent years but show it to be similar to strongly toxic years of the late 1980s, (b) show 3 year-groups, 1980s–early 1990s of high toxicity, mid-1990s–early 2000s of lower toxicity, and 2004–2005 similar to the early years, and (c) show temporal autocorrelation suggesting that processes carrying from year to year are important in controlling toxicity. Multivariate statistics then group stations according to interannual co-variability. Resulting station-groups show strong regionality consistent with known hydrographic and circulation patterns and identify stations that differ from their neighbors. Annual means calculated within station-groups provide 21-year time series that are cross-correlated with concurrent monthly environmental metrics of Gulf of Maine oceanographic conditions calculated from field measurements, satellite data and numerical circulation model hindcasts. These analyses provide three main results. First, toxicity time series show an overall pattern of elevated magnitudes and increased duration in the 1980s, minimum values in the mid-late 1990s and then increasing again in the 2000s. Second, only station-groups geographically located in the western portion of the study area have correlations with any tested environmental metric. Third, toxicity in many western station-groups was positively correlated to interannual variability in early season (April–May) wind stress driving onshore Ekman transport, negatively correlated with summer (June–July) wind stress driving offshore Ekman transport and negatively correlated with summer (June–July) cross-shelf surface temperature gradients indicative of relatively warm coastal surface temperature patterns, consistent with patterns expected from the wind transport correlations. The data do not show significant correlation between station-group toxicity and along-shelf temperature structure (an indicator of fronts and alongshore flow connection), river discharge, surface temperature anomalies, modeled surface salinities or alongshore current velocities. These data provide a quantitative summary of Maine coastal toxicity over 21 years, a spatial toxicity geography and isolate dominant environmental forcing responsible for interannual variability.