NUTRIENTS & LOWER FOOD WEB

Rapporter: Suzanne Levine, UVM

 

The following list of research priorities for 2005-2010 was developed by a team of watershed scientists and linmologists active in the Lake Champlain Basin and an outside reviewer.  It is based on the list developed by a similar group in 1999. Each of the previously issued priorities was reviewed to assess progress to date, and a decision was reached as to whether it should be retained as a continued priority, modified, or dropped (either because enough progress has been made to lower its significance or because other issues now are deemed more critical).  That the 1999 list has not been revised in a major way indicates that experts in the fields of nutrient dynamics and lower food web analysis have a good sense of what needs to be done, but have been held back by inadequate funding.

 

The order of the items listed below only roughly reflects the perceived importance of the research priorities within the two categories.

 

Nutrient Related Priorities

 

1. Urban and suburban nutrient inputs to Lake Champlain continue to grow. To manage runoff from urban/suburban areas, we need to know the relative importance and magnitudes of the various possible sources of P and N. Sources requiring investigation include lawn fertilizers, pet manure, streambank erosion caused by hydrologic changes, soil erosion at construction (and other disturbed) sites, commercial wastes, and dust and grime running off impervious surfaces. Analysis of stormwater runoff in the Basin has become a priority due to new regulations (e.g., Watzin et al., Bowden and McIntosh), but these efforts have not yet sought to pinpoint the origin of substances reaching drains. The best approach to source assessments might be a combination of literature review and focused field studies.

 

An assessment of the impact of P addition to city drinking water (to prevent pipe corrosion; about 500 µgP/L) should be made. While P is removed at the municipal treatment plant, some does not return to the plant (e.g., lawn watering will add this P to the landscape). We might look into alternatives to ZnPO4 as an anti-corrosion agent. If P in city water could be reduced, the P removal capacity of treatment plants might be improved.

 

There should be an assessment of the amount of P in wastewater associated with dishwasher detergents and other cleaning products not included in laws related to the P content of detergents for household use.

 

2. There is continued concern regarding how effective the non-point source management practices that are in use (agricultural BMPs mostly) have been in reducing P and N loads delivered to streams. Fertilization of fields has resulted in P and N storage on the landscape. We should know the extent to which this has occurred and understand how storage might affect later runoff loads. We need to know about the sustainability of our short-term fixes, and whether changing agricultural practices (e.g., larger farms) require altered approaches. New methods of agricultural waste treatment have been introduced.  The effectiveness of these practices should be determined to assist farmers in deciding whether to implement the changes.

 

Projects investigating these issues have begun during the past five years, and should be continued. These include assessment of BMP implementations in the urban watershed of Englesby Brook and along an agricultural section of Little Otter Creek in Vermont with regard to impacts on sediment, N and P loads. In Quebec, programs have been initiated to assess the effects of created wetland, riparian zone management, structural runoff control and subsurface drainage systems on phosphorus mobility.  Quebec scientists also are measuring P storage in soils. 

 

 

3. Nitrogen loading to the lake and its internal cycling need to receive more attention, given that the algae in the lake appear to be N as well as P limited at times, and given the possible importance of N availability in determining whether blue green algae are present. The monitoring program should be expanded from the current practice of TN measurement to analysis of specific important N forms (especially ammonium and nitrate).  Analysis of N storage in sediments, ammonium flux from sediments, denitrification and N fixation would aid greatly in understanding the N dynamics of bays with N shortages for algae (St. Albans and possibly Missisquoi Bays). TN data for the lake and its inflows might be used to assess the amount of N retained by the lake.

 

N inputs to the Basin have been increased by the nitric acid in acid rain. We should determine by how much. Is this source substantial relative to agricultural and natural sources? (There project overlaps with the goals of the Atmospherics group.)

 

It also would be worthwhile to assess the impact that our P reduction practices in the watershed have had on N runoff.

 

 

4. We need to know more about internal sources of nutrients in the lake, and how they compare in magnitude with nutrients entering in tributaries.  This is particularly important with regard to modeling response times to nutrient input reductions such as those now being implemented in the Missisquoi watershed.  Phosphorus flux from St. Albans Bay sediment was studied by HydroQual and the Univ. Maryland in the mid- 1990s. UVM researcher Greg Druschel has renewed this effort with more emphasis this time on understanding controls.   Similar work should be undertaken in Missisquoi Bay where P concentrations are very high and blooms especially problematic.  In addition, N flux should be assessed. The N:P ratio of the returned nutrient may influence nutrient limitation type and thus phytoplankton composition.  Variables that may affect P and N flux from sediments include sediment concentrations, redox chemistry, organic content, pH, bioturbation and macrophyte translocation. Measurement of nutrient concentrations inside and outside of macrophyte beds may provide insights into the role macrophytes play as P and N sources (or sinks). 

 

Lake Champlain's large and persistent seiche may be important in bringing nutrient from deep waters to the epilimnion during summer stratification. A cooperative study between physical limnologists (who would measure water exchange) and biogeochemists (who would measure N and P concentrations at different depths) could be very productive.

 

 

5. We should know more about the bioavailability of the different forms of N and P coming off the land and entering the lake. Not all forms are equally available; in particular, particulate forms may sediment without contributing to algal productivity. Dissolved organic nutrients may also be only sparingly available. This goal might be achieved through a combination of more detailed N chemistry (measuring ammonium and nitrate), bioassays for P availability, and occasional fractionations of dissolved P through ultrafiltration (which separates molecules by molecular size). The last would indicate how much dissolved P is present as large organic molecules versus phosphate (the standard assay for phosphate overestimates its presence by cleaving phosphate from organic substances; this overestimation was as much as 10 fold during the

LaPlatte River study of Hoffmann et al.(1996).

 

6. Monitoring programs for nutrient chemistry in the lake should continue, as they permit assessment of the effectiveness of nutrient reduction programs, warn of major changes in lake functioning, and provide information needed to understand and predict algal dynamics. The VT DEC has done an excellent job of making data readily available to researchers, and we hope that this openness will continue.  We suggest adding ammonium and nitrate analysis to the suite of nutrient fractions measured.

 

Probes and data loggers are being developed that can measure chemical concentrations and physical parameters continuously over time and transmit data to a lab computer. Use of these probes might be considered for installation in Lake Champlain.  Reliable probes exist for conductivity, pH, light transmissivity, and plant pigments (see 4 in the next section).  Nitrate and ammonium also can be monitored with probes but their sensitivity often is too low for lake waters. No probe exists for P.  Where probes would work particularly well would be in lake sediments where nutrient concentrations are higher. Nitrate, ammonium, pH, temperature and redox chemistry could be followed over time.  The measurements might be useful in estimating rates of nutrient efflux and thus predicting increases in algal biomass.

 

 

Lower Food Web Related Priorities

 

1. It would be useful to have a long-term record of the trophic state of Lake Champlain to better appreciate how much the lake has changed in recent decades (if it has really changed) and to have a baseline against which to gauge the effectiveness of management efforts.  Cores including sediments from the past two hundred years of population growth around the lake might be analyzed for a suite of well-established indicators of trophic state, including paleopigments, stable isotope ratios, algal and zooplankton microfossils, organic matter accumulation and nitrogen and phosphorus storage.  The record might be useful in revealing not only baseline conditions, but also how the lake has responded to previous changes in land use, the construction of causeways, and biological invasions.  The results might confirm or put to rest some Lake Champlain folklore (e.g., that St. Albans and Missisquoi Bays were once substantially more oligotrophic water bodies; that the diatoms that now dominate in the Main Lake during spring overturn dominated throughout the year in the past; that the placement of causeways across the entrance to Missisquoi Bay contributed to eutrophication, etc.).  This project might serve other purposes as well, such as documentation of changes in sedimentation rate, inputs of toxic pollutants, food web structure, water circulation or climate.  While coring at 10 or more sites around the lake is desirable, priority areas for trophic state analysis are the Main Lake, St. Albans and Missisquoi Bays.

 

2. There has been growing concern about blue green algae in Lake Champlain due to documentation of microcystin and anatoxins production and occurrence of dog deaths. Identification of the factors that promote bloom development and the release of toxic substances is a continuing priority. The approach should be three fold, including laboratory studies of the environmental physiology of the major species involved, empirical analyses relating trends in algal biomass, dominance or toxin production to environmental variables (such as nutrient and light levels, resource ratios, grazing, turbulence level, temperature and pH), and experimentation to test hypotheses.  Experiments in in-situ mesocosms are desirable but would be difficult in Missisquoi and St. Albans Bays due to heavy waves. They could be done in smaller lakes that have similar algal communities (e.g., Shelburne Pond).  Watzin et al. currently are involved in the monitoring of blooms and their toxicity and have been seeking relationships. 

 

It should be recognized that not just Lake Champlain but a number of small Vermont lakes are dominated by blue green algae in summer.   The State needs to track blooms in these lakes as well as in Lake Champlain so that swimmers and other users can be warned of unhealthy situations (see 5). 

 

3. We need to know more about controls on algal biomass, productivity and species composition in Lake Champlain in general. Levine et al (1997, 1999) have shown that phytoplankton in the Main Lake are not severely phosphorus limited.  Other variables such as nitrogen and light availability or grazing pressure limit at least some common species at times.  Better understanding of controls would improve management.  For example, targets for N and P inputs might be set to favor green algae and diatoms rather than blue green algae.

 

The large seiche in Lake Champlain (13 m amplitude on average with a 4 day cycle) probably has major impacts on nutrient availability and the light regime of phytoplankton in the lake.  These impacts (changes in rates of primary production, the dynamics of competition, and vulnerability to grazing as epilimnion depth changes) should be assessed through collaborative studies involving physical and biological limnologists.

 

4. Continued monitoring of phytoplankton communities throughout the lake is critical.  The biweekly sampling of 14 sites by the NY DEC should be continued. An effort should be made to analyze samples within a few months (several years of data were lost due to delayed counting and poor sample preservation during the 1990s), and data should be readily available to all local researchers through web posting.

 

New methods of monitoring that provide more detailed assessment of temporal and spatial trends in phytoplankton dynamics should be explored and tested. Satellite (or aerial) remote sensing can be used to obtain a more complete assessment of the spatial distribution of chlorophyll in the lake than obtainable through point sampling. Daily satellite images would allow for assessment of temporal trends (to the extent that clouds do not interfere with the assessment). In-situ probes to assess chlorophyll a and other pigment concentrations now exist and could be installed at representative points within the lake to continually track changes in algal biomass.  A combination of phycobilin and chlorophyll probes would provide information on the relative importance of blue green algae in the phytoplankton community.

 

6. Lake Champlain’s planktonic microbial foodweb (the part of the foodweb involving bacteria, protozoa and zooplankton) is very active during the summer (Levine et al. 1999). A substantial amount of the energy and nutrients reaching zooplankton and subsequently fish passes through this foodchain. We should know more about the lake’s microbial foodweb and how zebra mussels, which feed largely on bacteria and small algae, are affecting it.

 

Study of microbial processes in sediments is critical to understanding nutrient fluxes from this medium.