Project 9: Results 9/2000

The Impact of a Recurrent Coastal Plume on
Phosphorus Dynamics and Production in Lake Michigan

PIs: J. Cotner and T. Johengen

Overview of Research Activities:

This research project represents one of twelve separately funded components of the parent proposal entitled; The Impact of Episodic Events on the Nearshore-Offshore Transport of Biogeochemically Important Materials in the Great Lakes (EEGLE). Our project is designed to examine the influence of major episodic events on phosphorus (P) availability and dynamics in the southern basin of Lake Michigan. Efforts will focus on a recurrent coastal plume that develops in the spring. Understanding the impact of episodic events on P availability has important implications for ecosystem structure and function because primary productivity in this system is P-limited.

The goal of this project is to determine the importance of the recurrent coastal plume to transport, composition and biological availability of P. To achieve this goal, we will fulfill the following objectives: (1) To determine the potential sources, distributions and fluxes of P in the recurrent coastal plume and (2) To determine the impact of P associated with the recurrent coastal plume on phytoplankton and bacterioplankton production in Lake Michigan. Specifically, we will examine the impact of these suspended particles on microbial (heterotrophic bacteria and phytoplankton) community productivity and nutrient limitation through a combination of bioassays (growth rate measurements, alkaline phosphatase activity), nutrient chemistry and stoichiometry, and P regeneration rates.

Year one of the study (7/97 - 6/98) was defined as a pilot year and was designed to allow for the development of field sampling strategies, experimental designs, and new instrumentation. These tasks were accomplished in conjunction with all the investigators from the other projects through a series of workshops and research cruises. We participated in five major research cruises on Lake Michigan between January and May of 1998. All samples have been processed and analyzed from year 1 field surveys and experiments and were submitted to the project's database. As part of this project, a new nutrient analyzer was purchased and brought on line. A post-doc was hired through the University of Minnesota and began working in June 1998.

A late summer cruise was conducted in the start of year 2 (7/98 - 6/99) to measure bacterial production, nutrient concentrations, phosphorus cycling, and indicators of P-deficiency at our established transect sites throughout the southern basin. We are attempting to establish whether the effects of the plume can be observed in phosphorus dynamics and heterotrophic production throughout the summer and periods of maximum production. Five research cruises were conducted from January - June of 1999 to survey the lake during pre-, plume-, and post-plume conditions. All samples have been processed and analyzed from year 2 field surveys and experiments and were submitted to the project's database.

Year three activities (7/99 - 6/00) built upon results and strategies developed during 1999. Year three was the last full-scale field year, and cruises were again conducted to survey the lake before, during, and after the resuspension plume event. During the cruises we conducted surveys of nutrient concentrations, nutrient limitation, bacterial abundance, and total suspended material. At selected sites we performed experiments on bacterial production, nutrient uptake kinetics and equilibrium reactions. Nine cruises were carried out during this field year. In addition to the cruises, a muli-investigator (cross-project) experiment was conducted using mesocosms to examine the individual and synergistic effects of riverine inputs and sediment resuspension on nutrient dynamics, bacterail production, and phytoplankton production.

All hands planning meeting were conducted in October 1998 and October 1999 to coordinate sampling and research activities and to present results to a review panel of the NOAA/COP and NSF/CoOP funding agencies. We have made multiple presentations of our results at professional scientific meetings, at project workshops, and at student research symposiums. In addition we have published several manuscripts, and have a significant number in review or in preparation.

Results and Perspectives:

Hypothesis 1 - The recurrent coastal plume and/or resuspension of bottom sediments generates an increased inventory of total P and bioavailable P in the water column. This increased inventory is important to the development of the spring phytoplankton bloom.

P in the water column increases, mostly as particulate P
P-turnover times and alkaline phosphatase assays suggest that the particles may actually be sorbing P from the water, making it less available to plankton
Rivers are an important source of dissolved P and organic matter to nearshore plankton
Rivers also an important source of photo-labile dissolved organic matter.

Hypothesis 2-Heterotrophic bacteria function as "scavengers" of P in the coastal plume and maintain P availability in the water column.

Bacterial biomass increased substantially during the 1998 resuspension event
Carbon and other nutrients that are loaded into the water column from the sediments and rivers stimulate bacterial production at this time
Bacteria are an important sink for P

Hypothesis 3- The inventory of TP and available P within the plume decreases as the plume ages, as a result of biotic incorporation and settling.

Regions with elevated total P levels during a resuspension event, exhibit significantly lower P levels on follow-up cruises.
Post-plume total P levels are lower than winter, pre-plume levels, and regions within the plume show a greater decline than non-plume affected regions.

Hypothesis 4- The bioavailability of P on the particles is governed by (a) equilibrium with solid Ca-PO4 phases; (b) dissolution kinetics of CaPO4 phases; and/or (c) adsorption/desorption kinetics of PO4 on the Fe and Al oxides of the particles.

We have not addressed this hypothesis because our budget was cut.

Hypothesis 5- Water column P regeneration is more efficient during development of the recurrent coastal plume, minimizing losses to the sediments at this time.

Bacteria take up a large fraction of available P during resuspension events
They do not sink and may stimulate planktonic production by retaining P in the water column through the unstratified and stratified seasons.
This idea will be examined further by examining interannual variability in resuspension intensity, bacterial biomass and spring productivity.

Interactions:
There are approximately 40 principal investigators from 30 different universities, institutions, and governmental agencies collaborating on this project. Principal investigators interact through a series of annual workshops and collaborative research cruises. Workshops are held to coordinate research objectives, plan field activities, and share results. Daily interactions and information exchange are conducted through the project's web site and group mailing lists.

T. Johengen has served as the project coordinator for developing the field sampling program for the biological/chemical survey cruises and has served as the chief scientist for most of these cruises.

EEGLE-KITES collaborative cruise, June 2000. We worked with Sarah Green of Michigan Tech to examine in situ photo-oxidative versus biological degradation of organic matter in Lake Superior. Work related to this project was recently written about in the Minneapolis Star Tribune. .

Other Funding/Support:
This project is financially supported by the NSF CoOp and the NOAA COP programs. In addition, the U.S. EPA has contributed greatly to the project in terms of research vessel support and the funding of collaborative research projects that enhanced the scope of the EEGLE project.

Degrees Earned:
Simba Blood, B.S. Department of Ecology, Evolution, and Behavior, University of Minnesota.