Levels of Inorganic and Total Phosphorus in Lake Sediments as Related to Other Sediment Parameters Julian D. H . Williams,’ John K. Syers,2 Surendra S. Shukla, and Robin F. Harris Dept. of Soil Science, University of Wisconsin, Madison, Wis. 53706
David E. Armstrong Water Chemistry Laboratory, University of Wisconsin, Madison, Wis. 53706
The levels of inorganic P in 16 noncalcareous and 9 calcareous surficial sediments from 14 Wisconsin lakes were closely related to the amounts of short-range order hydrated iron oxides extractable by acid ammonium oxalate or neutral citrate-dithionite-bicarbonate (CDB). The Fe,/P atomic ratios of the “short-range order Fe-inorganic P complex,” resulting from the sorption of orthophosphate by hydrated F e oxides, varied between 5 and 10 for most sediments, although ratios exceeding 20 were obtained for six noncalcareous sediments. Variations in the amount of the complex accounted for most of the difference in total F e and total P between sediments. Oxalate- and CDB-extractable forms of AI were associated with organic P but not inorganic P ; organic P was also associated with organic C. Weak statistical relationships between Mn and inorganic P were accounted for by Mn-Fe interrelationships. Calcium carbonate (30-60 by weight of the calcareous sediments) was not directly related to any P parameter.
T
he onset and reversal of P-induced eutrophication of lakes are ultimately dependent on the rate of P input into the lake system (Vollenweider, 1968). Lake sediments, however, probably influence this process by virtue of their capacity to take up and release P under changing limnological conditions. Simple calculations indicate that the total quantity of P present in the uppermost 1 cm of sediment frequently greatly exceeds that present in the overlying water column, but little is known about the proportion of sediment P that can undergo exchange with lake waters and the rate at which such exchange occurs. By analogy with unfertilized terrestrial soils (Syers and Walker, 1969; Williams and Walker, 1969), it may be postulated that sediment P falls into four categories: inorganic P present as orthophosphate ions sorbed on the surfaces of P-retaining components (“nonoccluded P”), inorganic P present as orthophosphate ions within the matrices of Pretaining components (“occluded P”), orthophosphate P present in discrete phosphate minerals such as apatite [Calo(PO&Y2, where X = OH, F, 1/2C03]and vivianite [Fea(Pol)?.8H20] (“discrete P”), and organic esters of phosphoric acid (“organic P”). Inorganic polyphosphates may be present in sediments in localities where pollution by detergents is severe.
Present address: Canada Centre for Inland Waters, Burlington, Ontario, Canada. To whom correspondence should be addressed.
Because the nonoccluded inorganic P fraction is by definition in more intimate contact with the surrounding aqueous phase than the other categories of inorganic P, it may be anticipated that this fraction responds much more readily to changes in limnological conditions than occluded P or discrete P. Exchange of P between the solid and solution phases would be controlled by adsorption and desorption reactions. Preliminary results (Li et al., 1970) indicate that a high proportion of sediment inorganic P exchanges with 32Pwithin a few hours, indicating that much of the sediment inorganic P exists in nonoccluded forms. It is well established that secondary Fe- and AI-containing components are involved in the sorption of inorganic P by terrestrial soils. In particular, amorphous Fe and AI hydrated oxides and amorphous aluminosilicates (Gorbunov et al., 1961), all characterized by numerous hydroxyl groups in surface positions, readily react with orthophosphate by “exchange adsorption” of hydroxyl for orthophosphate. Crystalline Fe and A1 oxides and 1,/1 phyllosilicates such as kaolinite have a much lower capacity to sorb P (De Datta, 1964). The amount of phosphate retained in nonoccluded form by calcium carbonate in calcareous sediments is difficult to establish because of the difficulty in distinguishing quantitatively between Ca-bound and Al- and Fe-bound forms of nonoccluded P in calcareous materials (Williams et al., 1971b). Orthophosphate is retained by Fe, AI, and Ca compounds in terrestrial soils (Chang and Jackson, 1958). It is well established that Fe and P are closely related in lake sediments (Wentz and Lee, 1969). Extractions with solutions based on Na or N H 4 oxalate and on Na citrate plus Na dithionite have been widely used in studies of terrestrial soils to differentiate among the forms of Fe and A1 compounds. The purpose of this paper is to relate the levels of inorganic and total P which have accumulated in a range of sediments taken from 14 Wisconsin lakes to the amount, forms, and reactivities of Fe, AI, and Mn components and CaCOl in the sediments. Materials and Methods Samples. Sediments were collected from 14 hard-water and soft-water Wisconsin lakes, chosen to represent a wide range of trophic states (Table I). Eight of the nine soft-water lakes are situated within 20 miles of the township of Minocqua, Oneida County, northern Wisconsin; the ninth (Devils) is in Sauk County, south-central Wisconsin. The five hard-water lakes are all in the southern half of the state. Three (Wingra, Monona, and Mendota) are situated within or are contiguous with the boundaries of the City of Madison, Dane County; the others (Delavan and Geneva) are in Walworth County. The trophic state, assessed by visual observation and by the persistence of dissolved oxygen in the hypolimnetic waters Volume 5, Number 11, November 1971 1113
Table I. Characteristics of Lakes and Samples Sample identification County Conditiona no.
Lake
Water depth at sampling point (m)
Sediment group
SOFT-WATER LAKES Devils Little John
Sauk Vilas
0-M E
Trout
Vilas
0-M
1 1 2 1 2 3
5 1 2 1 1
Crystal
Vilas
0
Content Plum Little Arbor Vitae Mi n ocqu a
Vilas Vilas
E M
Vilas Oneida
M variableb
Tomahawk
Oneida
0-M
14 6 6 32 33 26 15 17 11 3 7
A A B A A B
5
B A A A B
1 1 2 2
15 22
B A A A
3
10
B
3 5 6 1 3 1 1 2 1
3 3 3 22 22 13 15 11 21
C C C C C C C C C
9
HARD-WATER LAKES
*
Wingra
Dane
E
Mendota
Dane
E
Monona Delavan
Dane Walworth
E E
Geneva
W alworth
M
0 = oligotrophic, M = mesotrophic, E = eutrophic. M-E at location of sample 1, 0 - M at sample 2.
during the period of thermal stratification, varies greatly among both the hard-water and soft-water lakes. Crystal is the most oligotrophic lake, in which benthic plants grow in 17 m of water. Although most of Minocqua is oligotrophicmesotrophic, as in the region where sample 2 was taken, one bay of the lake, from which sample 1 was collected, shows vigorous algal growth and other signs of advancing eutrophication, presumably due to sewage from the township of Minocqua. Probably because of its great depth, 80 m or more in places, Geneva is the only hard-water lake which is not markedly eutrophic and this lake is classified here as mesotrophic. Eutrophication is perhaps most pronounced in the case of Wingra. Samples of the surface sediments were taken with an Ekman dredge and stored in the dark at 4OC, in airtight bottles. Where more than one sample was taken from the same lake, there was usually a marked difference either in the depth of the overlying water or the distance of the sampling site from the shore, or both. Sandy-textured sediments, however, were avoided with the exception of Crystal 2 . Eight samples taken along two perpendicular transects of Lake Wingra were very similar in composition and three samples were chosen for further study here. Analytical Methods. Weight of oven-dried material in unit weight of undried sediment was determined by drying 1114 Environmental Science & Technology
at 110°C overnight. Subsamples of sediment were freezedried and ground to
