Stream ecosystem research at the Hubbard Brook Experimental Forest started in the mid 1960s, and has included: surveys of invertebrate taxa; long-term studies of stream chemistry, temperature and hydrology; flux, cycling and mass-balance approaches to element cycling; and manipulative experiments to examine stream processes.
Many manipulative stream experiments have been done at HBEF to examine stream ecosystem function. An example is the acid addition experiment performed by Ronald Hall and colleagues in 1978 (Hall et al. 1980). They added dilute sulfuric acid to Norris Brook to examine stream response to acidification. The target pH was 4 from a background pH of 6. They observed increased streamwater concentrations of Al, Ca, Mg, and K, but no change in dissolved organic carbon, Na, NO3, NH4, and several other metals. They found an increase in invertebrate drift, lower insect biomass, and reduced numbers of emerging insects during the five-month continuous acid addition. Many macroinvertebrates, especially collectors, appeared to respond to the acidification by leaving the section during the first week of the treatment. Higher periphyton biomass and lower numbers of fungal hyphomycetes were measured following acidification.
There have been other solute addition experiments at HBEF to examine material cycling in streams. Meyer (1979) added phosphate to Bear Brook to examine mechanisms of phosphate retention. She found that phosphate was removed quickly from the water column by abiotic processes, and that phosphorus concentration in stream water is held in equilibrium by abiotic exchange. Richey et al. (1985) added ammonium, nitrate and urea to Bear Brook. They found that much ammonium and urea were removed from the water column and nitrified to form nitrate. Nitrate was not retained in the stream ecosystem.
More recent research by Gail Steinhart and others suggested that denitrification may be a much more important process in streams of the HBEF than had been supposed previously (Steinhart et al. 2001). Laboratory studies of denitrification potential suggest that rates of denitrification in Bear Brook may remove as much as 25-110% of the nitrate output in stream water during July - October. McDowell (1985) added DOC in the form of extracts of maple leaves to measure DOC uptake. He found rapid uptake of DOC, which was likely abiotic. Interestingly DOC uptake in a stream (Watershed 5) with much organic matter removed was similar to Bear Brook, which has a high organic matter standing stock, suggesting that microbial processes associated with organic matter is not important to DOC uptake in Watershed 5.
Recently, whole-stream manipulations have been used to test the effects of alkalinazation on stream ecosystems and to trace the biological pathways of carbon and nitrogen. To test the role of streams in processing calcuim lost from watershed soils, Hall et al. (2001) added 240 µeq Ca L-1 as calcium chloride to two second-order streams for two months. Sodium bicarbonate also was added to study the effects of simultaneous increases in pH and alkalinity. In the high-pH stream (Ca + bicarbonate), 10-50% of the added calcium was removed from the water column in the 80-m study reach, but after the addition, Ca was released very slowly from the sediments. In the low-pH stream (Ca only), less Ca was removed by the water column and desorption of Ca was not measured following the addition. The algal community in these streams was not measurably affected by changes in calcuim or pH but the addition of calcium chloride negatively affected the emergence of the stonefly Leuctra ferruginea.
More recently, Likens et al (2005) added caSiO3 (Wollastonite) to a study reach below watershed 1. They found that Wollastonite was very effective in buffering the acidity of the stream water for long periods.
McCutchan and Likens added 13C bicarbonate to Falls Brook for 7 months to estimate the contribution of algal carbon to the growth of stream consumers. Preliminary results from this study indicate that some mayflies (e.g., Baetis sp., Ameletus sp., and Heptageniidae) were strongly dependent on algal carbon as a food source.
Research on the Mirror Lake ecosystem, near the mouth of the Hubbard Brook Valley began in the 1960's. Limnological analyses of the lake have included studies of the physics, chemistry, biology, hydrology, and geology. Because of an added focus on the airshed and watershed of the lake, it was possible to study comprehensive air-land-water interactions. These studies are described in a book "An Ecosystem Approach to Aquatic Ecology - Mirror Lake and it's Environment" (Likens, 1985).