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TESTING PHOSPHATE AVAILABLITY IN THE SEDIMENT OF RAVENSWOOD, PRESIDENTS & NORTH PONDS ON THE CALVIN COLLEGE CAMPUS

Doug Wetzel, Matt Post, and Renae Postma | December 11, 1998 | Biology 345A

Introduction | Methods | Results | Discussion | Figures | Table


INTRODUCTION

As the least abundant of the nutrients in its usable form, Phosphorus is usually found to be the critical limiting nutrient factor in aquatic ecosystems. Because it acts as the limiting nutrient, the health of the system depends directly upon the concentration of phosphorus that is present at a given time.

Input to aquatic systems can be of natural origin or anthropogenic. Natural inputs can include: wastes and dead organic matter, erosion of surrounding substrate, animal excretion and quano. Fertilization, application of pesticides, household detergents, and accelerated erosional processes due to particular land uses, are some of the ways that humans contribute additional phosphorus into these systems.

In an aquatic system phosphorus input will follow one of two courses. The producers may utilize it as a nutrient, or it may precipitate out and become bound in the bottom sediments. In the latter form, it is unavailable to the system until acted upon by certain natural elements, such as mixing into the waters by weather disturbances or changes in water temperature or pH. Testing sediments for phosphorus can not only verify its presence and concentration, but it can also indicate input source and reflect its movement and distribution through the system.

As some level of eutrophophic condition is apparent in all three of the ponds, Ravenswood, Presidents, and North, the presence of phosphorus was highly suspect. Prior testing, done only on the waters of the ponds, had not revealed that presence to any extent. However, it was quite possible that any phosphorus bound in the sediment and made available by its return to the water column was likely uptaken and utilized rapidly by the local biota. Consequently, that its presence could not be detected in the water was not an indication of its absence. It became a reasonable course of action to test the bottom sediments.

We tested the bottom sediments and middle waters of these three ponds for two types of phosphorus, reactive and acid-hydrolyzable. Testing for reactive-phosphorus reveals dissolved and suspended orthophosphates. Fully hydrated, orthophosphates are the type most facilely utilized by producers in the system. However, it is also the least abundant type as it is quickly uptaken by pond biota and also precipitates out quite readily. Acid-hydrolyzable phosphates are dissolved and particulate condensed, and becomes bound in the sediments as a precipitate. It is not readily available to biota. Acid hydrolysis converts these particulate condensed phosphates into orthophosphates.

METHODS

Each pond was reviewed on site, maps were made (Figures 1-3), and projected testing designations marked, determined as optimal locations for discerning directional source of input and pattern of distribution within the system. Ravenswood samples (Fig. 1) were taken on September 29, 1998. Presidents (Fig. 2) was done on October 5, and North (Fig. 3) on October 1. Sediment samples were taken from the surficial layer of bottom sediment using either a grab or a coring auger and entered into containers. For record and comparison, undisturbed water samples were taken, entered directly into containers from middle water, at approximately .5m depth. Samples were transferred to beakers, then put in the oven for several days to provide a dry weight. When the sedi-ments were completely dehydrated, it was necessary to crush and pulverize them as they had condensed and hardened. From each site sample three 10g. portions were measured out and entered into Erlenmeyer Flasks containing 100mL of deionized H2O. They were then shaken and allowed to rehydrate while preserved by refrigeration. Presidents Pond samples were rehydrated on October 8, Ravenswood samples on October 29, and North's on November 16.

All procedures we followed for testing were in accordance with ASTM method 4500-P. For the reactive-phosphate tests, the rehydrated samples were run through a quick filter (Fisher P8, Fast/Course, 15.0cm) to remove sediment, and then reconstituted to its original 100mL measure. Following the procedure for acid-hydrolyzable phosphate testing, a strong nitric/sulfuric acid solution was added to each sample. The samples were heated and allowed to reflux for 90 minutes. Following digestion, each sample was run through a quick filter to clear it of sediment and then reconstituted to its original 100mL.

For colorimetry, the ascorbic acid colorimetric method was chosen. 8mL of color reagent were added to 50mL of each prepared sample. 10 minutes was given for color development. The samples were then run through a Guilford spectrophotometer set at 880nm in order to obtain the absorbance readings. Several standards were also prepared at times of testing to make a correction factor to convert absorbance into concentration in parts per million (PPM). The concentration found was that of the solvent water, so for sediment samples an additional correction was made to find the concentration of the actual sediment (Table 1).

Testing was done on Ravenswood samples on November 5, on Presidents for reactive-phosphates on October 22 and for acid-hydrolyzable phosphates on the 23rd, and on North Pond's samples on December 19.

RESULTS

Phosphorus inputs in Ravenswood and Presidents Ponds find their origin in human activity. Levels of phosphorus are greatest for Ravenswood at its inlet (Fig. 4, R2). A drain about 75 feet southeast of the pond supplies water to a creek that runs into the pond (Fig.1, R2). This inlet supplies most of the phosphorus to both of these ponds. Following the easterly flow from this inlet toward Presidents Pond, the acid-hydrolyzable levels decrease. The flow from this inlet also contributes to the highest levels of phosphorus found in Presidents pond, at the isthmus connecting the two ponds (Fig. 2,P2; Fig. 4, P2). Here the flow is sufficiently depleted for substantial precipitation of phosphorus to occur.

A drain southwest of presidents pond supplies water to a 50-foot long patch of marshland that flows into the pond (Fig.2, P4). Because the water first runs through the ground, the phosphorus contribution from this site is not as significant. (Fig.5, P4). Following the northerly flow from this inlet and the northeasterly flow from the isthmus where Ravenswood Pond connects, the acid-hydrolyzable phosphorus levels decrease.

Within North Pond phosphorus levels depend upon natural erosional processes and non-point sources. Agricultural fields were once maintained within the pond's watershed. Phosphorus can be traced to the fertilization and accelerated erosion from these fields. Direction of water runoff and large amount of vegetation in the water determines the levels of reactive phosphorus in the pond (Fig. 3). Since the pond does not drain rapidly, phosphorus can collect easily through precipitation. Consequently, acid-hydrolyzable phosphorus levels are high throughout the pond (Fig.6).

DISCUSSION

Ravenswood and Presidents Ponds differ considerably from North Pond. Unlike North, within Ravenswood and Presidents influx of phosphorus is from a discernable point source. Phosphorus contributions within these ponds are traceable to human inputs. By controlling these inputs and preventing their access to the ponds through the inlets, phosphorus levels throughout both ponds can be decreased. Several measures were suggested by the Calvin Environmental Engineering students, including a wetland area to intercept the first inch of runoff and provide natural filtration, with a sediment settling tank at its approach to remove a considerable amount of contaminants even before they enter the wetland. One feasible measure suggested to decrease the present concentrations was the option of dredging the ponds. An earlier suggestion by the team responsible for assessing the Nekton populations of the three ponds, was the introduction of larger predators to the systems in the pursuit of restoration by trophic cascade.

Within North Pond influx of phosphorus can be attributed to natural erosional processes and runoff, however, it is likely that the major source is still human activities, and that the erosional processes have been accentuate by land use practices. The direction of water runoff and large amount of vegetation in and around the pond determines the levels of reactive phosphorus within in the pond. Acid-hydrolyzable phosphorus levels are high throughout the pond, and give little indication of optimum locations for intervention. However, along the east side of North Pond the considerable slope there could be guarded by silt fencing and deter input from that direction. It might also work elsewhere. Since the water body is land locked, dredging could be a viable and desirable option for North Pond as it was for Ravenswood and Presidents. Following the improvement of conditions by dredging, a trophic cascade might be induced here as well, and may serve to stabilize the community.

WORKS CITED

Brower, James E., Jerrold H. Zar, and Carl N. VonEnde. 1998. Field and Laboratory Methods for General Ecology, fourth edition. McGraw-Hill, Burr Ridge, Illinois, USA.

Eaton, Andrea D., Lenore S. Clesceri and Arnold E Greenberg, editors. 1995. Standard Methods for the Examination of Water and Wastewater, 19th edition. American Public Health Association, and Water Environmental Federation, Washington D.C., USA.

Kormondy, Edward J. 1996. Concepts of Ecology, fourth edition. Prentice Hall, Upper Saddle River, New Jersey, USA.


FIGURES- click on any figure to view full size

Figure 1 Figure 2
   
Figure 3 Figure 4
   
Figure 5 Figure 6

 

Table 1. Measures of phosphate concentrations in each of the ponds tested on the Calvin College Campus

Concentrations are represented in parts per million.

Pond Location Designation

Reactive Phosphate (ppm)

Acid-Hydrolyzable Phosphate (ppm)

 

 

 

Ravenswood

 

 

R1s (middle)

1.5

6.6

R2 (inlet)

1.9

13.8

R3 (outlet-east)

1

2.5

R4 (crossflow)

1.1

5.3

R5 (north)

0.8

1.9

R1w (middle)--water

< 0.005

0.05

 

 

 

Presidents

 

 

P1s (middle)

0.12

12.4

P2 (inlet)

0.4

21

P3 (north)

< 0.005

2.3

P4 (south)

< 0.005

5.2

P1w (middle)--water

< 0.005

< 0.005

 

 

 

North

 

 

N1s (middle)

1.9

25.6

N2 (north)

1.9

27.3

N3 (south)

4

27.4

N4 (east)

3

27.2

N5 (west)

4.5

27.2

N1w (middle)--water

0.13

0.03