At that time the focus of how TASL data could be used shifted somewhat. Some predicted that, with better water clarity and less contamination in sediments and overlying waters, there could be increases in bird populations or changes in feeding areas. On the other hand, it was also recognized that once the nutrient-rich wastewater effluent discharge was moved from Deer Island to nine miles offshore, the reduction of nutrients in the harbor could reduce marine productivity and possibly lead to a reduction in the wintering population of water birds.

A superficial analysis of November TASL results over the past 20 years might suggest that the increased diversity and abundance of waterfowl observed in 1998 - 2000 could be due to cleaner effluent brought about by the beginning of secondary wastewater treatment in 1998. On the other hand, the reduction in bird numbers in the fall of 2001 could be seen as consistent with the concern that reducing nutrient discharges into the harbor would reduce bird populations (and other marine resources). Just the other day a fellow birder was bemoaning the lack of birds around Hull this year and damning the MWRA for making the harbor “too clean.”

Are the changes we have seen in bird populations really due to cleaner conditions? Let’s heed our project name—let’s Take A Second Look.

For many TASLers of the 1980s it was a standing joke that one could predict the weather for a TASL count well in advance: it was always going to be cold, very windy, and often with snow or rain showers in the air. The reduced visibility and rough seas that these conditions frequently produced were two reasons why some of the early counts resulted in such low numbers of birds. Weather conditions over the past several years have been anything but “traditionally TASL”. Milder, sunnier, calmer days are much more the norm. What is going on?

We have all heard of the specter of global climate change. Also we’ve heard of and seen the extreme impacts of El Niño or La Niña years. With so much focus on climate/weather issues researchers are now looking more closely at a cycle that regularly impacts our local weather—the North Atlantic Oscillation (NAO). The NAO is characterized by changes in atmospheric mass between the Arctic and sub-tropical Atlantic. An NAO index is calculated based on differences in sea level barometric pressure between Iceland and Portugal. It is thought to be a major determinant in New England winter weather and in both ocean and land productivity during the summer. Possibly most ecosystem processes are affected by it. For example, the record number of Atlantic Right Whale calves born last winter after several years of very low birth rates is believed to be linked to the effect that the NAO had on zooplankton (the food of the right whale) production in Cape Cod Bay. (There had been growing concern that pollution or too much inbreeding had caused the reduced birth rate, which if remained unchecked, would hasten the extinction of the species.)

For the Boston area a “positive” NAO index is believed to lead to milder, less stormy winters and, during the summer, to higher ecosystem productivity. The opposite applies for a “negative” NAO. In the late 1950s and 1960s the NAO index was consistently negative—possibly the source of many of our childhood memories of colder, stormier winters. The 1970s and 1980s were transition years with some positive and some negative years. (Example: The blizzard of 1978 occurred during a negative year.) With the exception of a sharply negative period around 1996 and a lesser drop in 1991, the last decade has tended to be positive. More recently, in January and February 2001 (mild, quiescent months) the NAO index was positive; in March (a stormy month) it was negative. From June to October it was neutral to moderately negative—region-wide marine productivity appeared to be lower than average this summer.

In a general way our TASL weather data has some interesting parallels with the NAO data. 1980 and 1983 appear to have been years when the NAO was moving into a positive state: Diversity and abundance of birds was relatively high. The opposite was true in 1982. More recently the lower TASL counts in 1996 and 1997 corresponded with a negative NAO index. Since 1997 the NAO index moved into an increasingly positive state. TASL counts have shown an unprecedented (for TASL counts) abundance and diversity of birds in and around the harbor.

A positive NAO index is thought to lead to increased phytoplankton (the foundation of the marine food chain productivity). Increased phytoplankton production in turn leads to increases in zooplankton. Monitoring data substantiates that there were increasing levels of marine phytoplankton from 1997 to 2000, especially during the fall. So do the SeaWiFS images. (SeaWiFS is a new satellite imaging system started in 1997 and capable of measuring worldwide ocean and terrestrial productivity.) The slides provides composite images for chlorophyll production at the ocean surface in Massachusetts Bay from September and October 1997 to 2001. September and October 2000 measurements of phytoplankton indicated an unusually large bloom in this area. Additionally, there were reports of abundant schools of fish (herring?) close to shore from Cape Ann to Boston. Remarkably, two Humpback Whales were also observed from shore in September, one off Salem and the other in Boston Harbor off South Boston. This was followed in November by the most abundant and diverse assemblage of water birds we have seen in the 20 years of TASL counts. The NAO index appears to have moved from neutral in August 2000 to sharply positive in September and October.

On the other hand, the NAO index during the summer and early fall of 2001 appears to have been neutral to negative. Monitoring data from around the region indicates lower than normal phytoplankton and zooplankton production, as might be expected from such a NAO phase. The SeaWiFs images appear to substantiate these decreases. Fall TASL count totals were lower than normal. Counts made from Deer Island during the recent Greater Boston Christmas Count indicated a big drop from the already low counts observed during the November TASL count.

What this suggests is that the year-to-year changes we are seeing may be to some degree caused by changes in the NAO index. Clearly, a more detailed analysis of temporal patterns of individual bird species and more detailed data on historical NAO phases than I currently have are needed to separate out any impact of the NAO from other possible impacts, such as changes in water quality in the harbor. That work is beginning.

Maury is co-organizer and data-compiler for the TASL winter censuses. He is employed by the Massachusetts Water Resources Authority (MWRA) as staff biologist.

last updated: 16 Feb 2002
url: http://www.gis.net/~szendeh/tchanges.htm