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Chemistry, Mineralogy, and Morphology of Lake Erie Suspended Matter
J. Great Lakes Res. 10(3):286-298 Internat. Assoc. Great Lakes Res., 1984
CHEMISTRY, MINERALOGY, AND MORPHOLOGY OF LAKE ERIE SUSPENDED MATTER
A. Mudroch
Environmental Contaminants Division National Water Research Institute Burlington, Ontario L7R 4A6
ABSTRACT. Suspended matter was collected from different depths at three stations in spring and summer, 1978, in Lake Erie. Chemistry, mineralogy, and morphology of the suspended particles were measured to investigate spatial and temporal changes. The determined elements (Si, Fe, Ca, K, Mn, P, AI, Ti, and Mg) were partitioned between inorganic and biological material and the majority of these elements were present in at least two different chemical forms. Flocculates > 3 Jlm composed of organic material and mineral fragments were common at the water surface and the middle of the water column. Mineral fragments < 3 Jlm were the major constituent of suspended matter at the bottom at the deepest sampling station (62 m). The concentration of the major components of the suspended matter, organic material, alumino-silicates, and calcite, varied significantly from spring until later summer. The high concentration oforganic material and the fluctuations ofcalcite conce/l? tration result from high rates of photosynthesis and respiration and temperature increases in spring and summer. ADDITIONAL INDEX WORDS: Organic matter, electron microscopy, X-ray fluorescence.
(Vollenweider et al. 1974) with abundant suspended particles derived from shoreline erosion, river inputs, aeolian material, autochtonous organic matter, and bottom sediment resuspension. The objective of the present study was to characterize the suspended matter in the lake by mineralogical and chemical composition and by investigation of the morphology and associations of individual particles in the water column.
INTRODUCTION Suspended matter is an important component of any aquatic ecosystem for its ability to transport contaminants and nutrients. It provides a very large surface area available to various chemical reactions which may invalidate many of the dilute solution chemistry equilibria. Past studies on suspended matter in the marine environment (Price and Calvert 1973, Deuser et al. 1983), river estuaries (Sholkovitz 1979), and in freshwater systems (Wright and Nydeggen 1980, Giovanoli et al. 1980) included investigation of the distribution, flux, and sedimentation pattern of suspended matter and temporal and spatial changes in its chemical composition. The knowledge of the character and composition of particles suspended in the water column are a necessary step for further understanding of pathways and cycling of contaminants in a lake. Many investigations were carried out in the past on limnological processes within Lake Erie (Herdendorf et al. 1974, Burns 1976). This lake is known to be mesotrophic to eutrophic in nature
MATERIALS AND METHODS Suspended matter was collected during 1978 in Lake Erie at three stations: Nos. 2 and 94 in May, August, and September and at No. 23 in May and September (Fig. 1). These three stations were selected for their following features: station 2 was located in the eastern basin of Lake Erie at a water depth of 22 m. Station 94 at a water depth of 14 m was the shallowest of the three stations and located in the central basin, and station 23 was at the deepest point of Lake Erie (62 m). At each station about 600 L of lake water were 286
287
LAKE ERIE SUSPENDED MATTER 43"f-~~-+-~~~1r'='=----+-~~~~~+-~~~~~+-~~~------J\1
Lake Erie 42°
•, (J
I?
83°
<:J
82°
81°
80°
79
FIG. 1. Sampling localities in Lake Erie.
pumped from a I-m depth, mid-water column, and I-m above the sediment-water interface into polyethylene containers and then centrifuged through a continuous-flow Westphalia separator. The collected suspended matter was then freeze-dried and homogenized in an agate mortar. Surface 0 to l-cm sediment was collected by subsampling a benthos core collected at each sampling station. The sediment was freeze-dried and ground to 189 p,m size (100 mesh) for further analyses. The quantity of suspended matter collected by centrifugation was less than 1 g. Consequently the X-ray fluorescence spectrometry technique described by Sholkovitz (1979), using deposited suspended matter on filter papers, was considered the only practical method for the quantitative determination of Si, AI, Fe, Mg, Ca, Na, K, Ti, and P. The problems involved in the X-ray fluorescence analyses of natural particulate matter deposited on a filter paper are mainly the grain size and the composition of individual particles. Holmes (1981) theoretically and experimentally examined the method and proposed several ways to deal with the problems, for example the use of standards and samples of a similar composition and grain size, or to correct for absorption and enhancement effects by equations containing relevant parameters. Camp et al. (1975) presented results of an interlaboratory study of element determination in simulated and real particulate samples. Thin layers of similar mass (200 p,g cm- Z) were prepared by resuspending the dry homogenized samples in 250 mL of distilled water and filtering
identical aliquots onto 0.45-p,m Nuclepore membrane filters. Geological reference samples used as standards were prepared by the same method. A Philips X-ray fluorescence spectrometer PW 1450 was programmed for the determination of major elements (Cr target, 50 kV/50mA, argon-methane gas, flow proportional gas counter, PET, TLAP, AND, and LIF crystals). Four samples were run simultaneously: the first sample was a drift monitor powder pellet, the second sample was a blank Nuclepore filter, and the others were duplicates of the same sample. Both calibration samples remained constantly in the analyzer. A computer program based on two standard methods using six geological standards was used for the calculation of major elements in samples. Precision of the analyses was determined by analyzing, in quadruplicate, major elements in six various geological standards and one lake bottom sediment sample obtained from station no. 2 in Lake Erie (Fig. 1). The Q- test was carried out to determine whether at the 900;0 confidence level any results should be rejected. All results were retained. Relative standard deviations were in the following range: SiOz 1-3%, Al z0 3 4-6%, Fe z0 3 3-10%, MgO 4-14%, CaO 4-8%, NazO 5-15%, KzO 3-8%, TiOz 3-10%, MnO 10-20%, and PzOs 5-20%. The limit of detection and the limit of quantitation were defined to determine if the measured values of samples on Nuclepore filters (SI) were significantly different from those found for blank Nuclepore filters (Sb)' With the exception of NazO (SI - Sb = 3a) and MnO (SI - Sb = 8a),
288
A.MUDROCH
Le., standard deviations in the region of less certain quantitation, the values for all elements were greater than lOa, Le., the level above which quantitative results may be obtained with a specific degree of confidence. To determine the accuracy of the applied X-ray fluorescence technique two standards (granite and syenite) were deposited on Nuclepore filters in duplicate and analyzed by the described method. The relative errors of the calculated concentration of both analyzed standards were as follows: SiOz 2070, Alz0 3 4070, Fe z0 3 5070, MgO 14070, CaO 6070, NazO 20070, KzO 8070, TiOz 10070, MnO 25070, and P Z05 20070. The accuracy evaluation should be interpreted bearing in mind that geological standard reference materials do not fully simulate natural samples such as bottom sediments and suspended matter. Qualitative determination of mineralogical composition of suspended matter was carried out by the powder X-ray diffraction technique using CuKa radiation. Analyses of surface sediment were carried out by X-ray fluorescence spectrometry using powder pellets (Mudroch 1977). Organic C was determined in suspended matter and surface sediments with a Leco carbo analyser. The investigation of morphology and chemical composition of individual particles in these samples was carried out with an AMR 1000 electron microscope (EM) equipped with an energy dispersive X-ray (EDX) spectrometer and a computerbased data processing system (Tracor-Northern, TN-ll). Identification of minerals from the EDX spectra was accomplished using the method of Mudroch et al. (1977). For the EM study about 100 mL of water from each sampling station and depth were filtered sequentially through 8-Jtm, 3-Jtm, and 0.45-Jtm pore size Unipore polycarbonate membranes. The membranes with deposited particles were dried in a dessicator, cut to appropriate size, and mounted on a specimen support stub using two-sided adhesive tape. To make the specimen surface thermally and electrically conductive, all prepared samples were covered by a 20-nm-thick layer of carbon. The carbon coating was performed in a vacuum evaporator at a pressure of 10- 5 torr. RESULTS AND DISCUSSION The mineralogy of the suspended matter showed a close similarity to that of bottom sediments (Kemp et al. 1976). X-ray diffraction revealed the pres-
ence of quartz, feldspars, calcite, illite, and small quantitites of chlorite, kaolinite, and dolomite. However, the quantities of each mineral at all three stations varied with sampling depth and time. Results of the quantitative determination of Si, AI, Fe, Mg, Ca, Na, K, Ti, Mn, P, and organic C in suspended matter and 0 to 1 cm surface sediment are presented in Table 1 as Jtg g - I dry matter. The concentration of organic C varied with depth and among the stations (Table 1) and diluted the inorganic components represented mainly by mineral particles. A gradual increase in organic C occurred in suspended matter near the bottom at stations 94 and 2 from May to September reflecting sedimentation of dead phytoplankton. The concentration of organic C was similar at all three sampling depths at the shallowest station (no. 94) in September due to the mixing of the entire water column. The organic C concentration was almost four times higher in suspended solids than in the surface sediment at this station in September, indicating decomposition and transport of organic matter in the water column or at the immediate sediment-water interface. At the deepest point of the lake (station 23) organic C was higher at the surface in May than in September, probably reflecting higher primary production at this area in spring than in later summer (Munawar and Munawar 1976). A significant decrease in organic C occurred toward the bottom at this station in both May and September. Similar concentrations of organic carbon were observed at a 30-m depth and at the bottom in May and September, indicating steady conditions below the thermocline during both months. The concentration of Al in suspended matter was almost two times higher at the bottom than at the surface at station 94 in May (Table 1). However, very little spatial variation was observed in Al at the same station in August and September. This station, located in the central basin, was the shallowest of all three sampling stations (water depth 14 m) and was not stratified in September. In May a greater than twofold increase in Al occurred toward the bottom at station 2 located in the eastern basin (water depth 22 m). This increase was still significant in August and September when the thermocline was about 3 m above the bottom. A more than twofold increase in Al was observed toward the bottom at station 23 (water depth 62 m) in samples collected in May and September. The concentration of Al in suspended matter was higher at the bottom at this station in September
289
LAKE ERIE SUSPENDED MATTER TABLE 1. Lake Erie, Suspended solids analyses station no. 2, Depth 22 m.
21 m
10m
1m
21 m
10m
1m
21 m
10m
1m
Bottom Sediment o to 1 cm (650/0 clay, 350/0 silt)
9.55
11.20
25.50
12.55
14.75
34.55
15.85
23.52
29.81
4.43
42.8 8.1 3.0 0.9 9.8 1.7 0.32 0.07 1.4
33.2 6.0 2.1 0.6 7.2 1.3 0.24 0.06 1.0
36.2 4.9 1.6 0.6 4.3 1.1 0.20 0.06 0.5
53.37 13.63 7.08 2.18 1.49 3.81 0.54 0.20 0.44
May Percent Organic C
September
August
Si02 53.1 45.4 24.9 41.1 39.9 28.5 A1 20 3 9.1 3.5 6.9 6.8 4.7 9.4 Fe20 3 3.8 2.4 1.4 2.9 2.3 1.6 MgO 1.0 0.5 0.6 0.6 0.6 0.7 8.1 6.7 CaO 8.4 9.8 5.3 9.6 2.4 K20 1.8 1.1 1.6 1.5 1.2 Ti0 2 0.36 0.34 0.14 0.28 0.27 0.16 MnO 0.09 0.08 0.05 0.08 0.06 0.05 P 20 S 0.9 0.8 1.4 0.9 1.0 1.3 Lake Erie, Suspended solids analyses station no. 23, Depth 62 m. May Percent
61 m
30 m
1m
61 m
30 m
1m
Bottom Sediment o to I cm (68% clay, 32% silt)
5.21
10.90
21.65
4.24
11.70
17.95
3.30
53.6 9.0 3.6 1.1 8.2 2.5 0.38 0.09 0.8
43.5 8.9 2.5 0.4 8.3 1.7 0.32 0.08 0.7
20.1 4.9 1.7 1.0 19.7 1.4 0.15 0.06 1.8
53.2 13.0 4.1 0.6 15.3 2.9 0.48 0.11 1.8
38.5 11.0 3.1 0.5 6.5 2.3 0.39 0.07 0.05
32.4 5.4 2.0 0.4 7.6 1.3 0.22 0.06 0.7
56.79 12.58 7.59 3.23 2.11 3.70 0.52 0.27 0.32
Organic C Si02 A1 20 s
Fe20 3 MgO CaO K20 Ti0 2 MnO P 20 S
September
Lake Erie, Suspended solids analyses station no. 94, Depth 14 m. May Percent Organic C Si02 A1 20 3 Fe203 MgO CaO Na20 K20 Ti0 2 MnO P 20 S
August
13m
6m
1m
13 m
6m
1m
13m
6m
1m
Bottom Sediment o to 1 cm (77% clay, 23% silt)
6.64
11.48
17.45
16.88
20.52
21.89
20.29
20.90
22.20
5.13
51.6 10.4 3.0 0.4 7.0 0.6 2.0 0.35 0.09 0.5
46.2 9.3 2.5 0.4 7.8 0.6 2.0 0.32 0.08 0.6
26.3 4.0 1.4 0.2 13.2 1.3 1.2 0.23 0.05 0.9
41.2 6.6 2.3 0.3 3.6 0.2 1.4 0.23 0.07 0.6
38.3 4.3 1.5 0.2 3.1 0.2 0.9 0.16 0.05 0.4
34.3 5.3 2.4 0.3 6.6 0.3 1.3 0.24 0.06 0.9
33.4 5.2 2.0 0.3 6.6 0.3 1.3 0.20 0.06 1.0
34.7 5.3 1.9 0.4 7.6 0.3 1.3 0.19 0.06 0.9
39.4 5.1 1.8 0.2 3.7 0.2 1.1 0.18 0.06 0.4
55.52 15.39 7.54 2.13 1.40 0.99 3.50 0.57 0.13 0.4
than in May and was similar to the Al concentration in the bottom sediment (Table 1). Spencer and Sachs (1970) have shown that Al in the particulate matter from the Gulf of Maine was almost entirely associated with alumino-silicates.
September
Our electron microscopy investigation showed that Al occurred exclusively in alumino-silicates in suspended matter in Lake Erie. The Al concentration therefore represents the quantity of suspended alumino-silicates.
290
A. MUDROCH
TABLE 2. Element ratios in Lake Erie suspended matter.
Station 2
Si/AI
Ca/Al
K/AI
Fe/AI
Mg/Al
Ti/Al
May
1m 10m 21 m
7.05 4.99 5.66
2.77 0.92 0.87
0.33 0.20 0.26
0.40 0.27 0.41
0.18 0.06 0.11
0.04 0.03 0.04
August
1m 10m 21 m
6.06 5.87 5.90
2.05 0.99 0.76
0.26 0.22 0.23
0.36 0.34 0.42
0.15 0.09 0.10
0.03 0.04 0.04
September
1m 10m 21 m
6.32 5.48 5.29
0.89 1.20 1.22
0.23 0.22 0.22
0.34 0.35 0.38
0.13 0.10 0.12
0.04 0.04 0.04
Sediment
3.84
0.11
0.28
0.52
0.16
0.04
Station 23
SiiAl
Ca/AI
K/Al
Fe/AI
Mg/AI
TiiAI
May
1m 30 m 61 m
4.04 4.97 5.92
3.96 0.92 0.91
0.30 0.20 0.27
0.35 0.28 0.41
0.20 0.06 0.12
0.03 0.04 0.04
September
1m 30 m 61 m
5.94 3.48 4.07
1.39 0.58 1.12
0.24 0.22 0.22
0.37 0.29 0.32
0.08 0.05 0.05
0.04 0.04 0.04
Sediment
4.51
0.17
0.29
0.60
0.26
0.04
Station 94
SiiAI
Ca/AI
K/AI
Fe/Ai
Mg/AI
Ti/AI
May
1m 6m 13m
6.52 4.95 4.93
3.28 0.84 0.67
0.30 0.22 0.20
0.36 0.27 0.29
0.05 0.04 0.04
0.06 0.03 0.03
August
1m 6m 13 m
6.48 8.77 6.19
1.24 0.73 0.55
0.25 0.22 0.22
0.47 0.35 0.35
0.07 0.07 0.05
0.05 0.04 0.03
September
1m 6m 13m
7.69 6.46 6.37
0.73 1.43 1.26
0.23 0.24 0.25
0.35 0.35 0.39
0.05 0.08 0.07
0.04 0.04 0.04
3.60
0.09
0.22
0.48
0.14
0.04
Sediment
Mineralogical investigation by X-ray diffraction showed that Si was present in suspended matter in alumino-silicates and quartz. The mean Si/Al ratio of shales is about 3.0 (Krauskopf 1967). The SilAl ratios were 3.84, 4.51, and 3.60 in the surface sediment at stations 2, 23, and 94, respectively (Table 2). The bottom sediment usually contains more detrital quartz relative to alumino-silicates than the suspended matter (Price and Calvert 1973). Therefore, the latter should have a lower Sil Al ratio. This was observed at station 23 at the surface in May and at the bottom and the middle of the water column in September. However, the
SilAl ratio was higher in the rest of suspended matter samples collected at station 23 and in all samples collected at stations 94 and 2 than in the surface sediment (Table 2). Electron microscopy showed that the excess of Si was of biogenic origin appearing mostly in diatom frustules. Biogenic Si concentrations were highest at the surface at stations 93 and 2 in May and at station 23 in September, reflecting seasonal change in biological production and species composition at each sampling locality (Munawar and Munawar 1976). The electron microscopy, X-ray fluorescence, and X-ray diffraction analyses indicate the presence of three
291
LAKE ERIE SUSPENDED MATTER
different forms of Si in suspended matter: biogenic, and that as quartz and alumino-silicates. The Ca concentration and Cal Al ratio are shown in Tables 1 and 2, respectively. According to Goldschmidt (1958), a carbonate-free, finegrained sediment has a Cal Al ratio of 0.033. This value was exceeded many times in suspended solids collected in Lake Erie, because the soils in the drainage basin are mainly derived from sedimentary rocks where calcite is a common mineral. The Cal Al ratio in the sediment was 0.11, 0.17, and 0.09 at stations 2, 23, and 94, respectively. The ratio was much higher in suspended matter, particularly at the I-m depth at all three sampling stations in May. Mineralogical analyses showed that the majority of Ca in all samples was present as calcite precipitated in the lake or as detrital grains introduced from the drainage basin. Precipitation of CaC0 3 is a significant process in the Great Lakes. Strong and Eadie (1978) have shown that precipitation is greatest during periods of thermal stratification, and Rossmann (1980) found that the composition of the suspended solids in southeastern Lake Michigan is dominated by authigenic calcite in August and September. The temporal increase in Ca concentration in suspended matter indicated the presence of precipitated CaC0 3 • Precipitation at the three sampling stations was highest at the surface during May when the surface water temperature started to increase. In September the CalAl ratio increased at the bottom, most likely showing the sedimentation of precipitated CaC0 3 • The variations of the Cal Al ratio between the stations in August and September may reflect the changes in pH introduced by the primary producers followed by calcite precipitation. High correlations of Al with K and Ti in suspended matter indicated association of these two elements with alumino-silicates (Fig. 2). The X-ray diffraction and EM investigation showed that the alumino-silicate particles were mostly clay minerals, biotite, and feldspars. The KI Al ratio in suspended matter was similar to that found in the sediment at stations 2 and 23 (Table 2). Krauskopf (1967) gave a KI Al ratio of 0.29 for an average shale. The EM investigation indicated that particles > 2 JLm had a higher feldspar I clay ratio compared to the < 2-JLm particles, as well as a higher KI Al ratio. Variable quantities of feldspars in suspended matter most likely contributed to the small variations in the KI Al ratio at each station. Analyses of particles of biological origin by EDX showed that some K was also incorporated in algal cells
.5 .4
,
0'" .3
F
~
•
.2
0 0
2
4
•
•
e• • •
.:e
r=0.94
• • ••
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. •• • ~
8
6
10
12
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... . • r =0.94
0 0
10
12
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FIG. 2. Relationship between K, Ti, and Al in suspended matter.
and may affect the KI Al ratio, especially in suspended matter at the surface. A fraction of Mg was held in clay minerals. However, no significant correlation was found between Mg and Al because Mg was partly present in dolomite particles of detrital origin. According to Krauskopf (1967), the average shale MgI Al ratio is 0.17, which is close to that found in the Lake Erie fine-grained sediments (majority of < 2 JLm size particles). Poor correlation between Al and Fe in suspended matter (correlation coefficient = 0.145, no. samples 24) suggested that the occurrence of suspended Fe is not controlled by alumino-silicates and that Fe is present in other forms. The Fe was found in almost every individual particle of organic or inorganic origin examined by EMI EDX. The Fel Al ratio in 0- to l-cm surface sediment was almost two times higher than in suspended matter and reflected the migration of Fe in the sediment followed by precipitation at the sediment-water interface. Data of Kemp et al. (1977) show a significant decrease of the Fel Al ratio in sediment below the 0 to l-cm layer. Manganese behaved similarly to Fe, indicating little incorporation in alumino-silicates. Manganese concentration was higher in the surface sedi-
292
A.MUDROCH
ment at each sampling station than that in suspended matter reflecting similar migration and precipitation as Fe (Sozanski and Cronan 1979). The TilAl ratio was similar in suspended matter and sediment at each sampling station (Table 2). The EM investigation showed that Ti was present mainly in alumino-silicate fragments. The concentration of particulate P was highest at the surface at all three sampling stations in May (Table 1). It remained highest at the surface at stations 94 and 2 in August; however, a significant decrease of P was observed at the surface followed by an increase at the bottom at all three sampling stations in September. No relationship was found between organic C and P, and between Fe and P. Significant correlation existed between Ca and P concentration (correlation coefficient = 0.812, no. samples 24), suggesting a possible coprecipitation of P with calcite similar to that observed in other studies (Rossknecht 1980, Murphy et at. 1983). However, seasonal variations in P concentration also indicated association with organic material. The P concentration in suspended matter was up to three times higher than in the surface sediment, indicating regeneration of P within the water column. The results show that organic material is one of the major components of suspended matter in Lake Erie. The phytoplankton bloom is obviously responsible for the high levels of organic matter in the epilimnion compared to the hypolimnion of the deep water in eastern Lake Erie. The results further indicate that the concentration of organic matter in the epilimnion increases with decreasing distance from the shore. Nriagu et at. (1981) suggested that organic matter represents an important vehicle for metal transport to lake sediment. However, after decomposition of organic matter in the water column the associated metals may become chemically and biologically available. On the other hand, metals will most likely become associated with sediment particles after decomposition of organic matter at the sediment-water interface. Consequently, the decomposition and settling rates of organic matter are important pathways of the metals cycling in the lake. Another important component of suspended matter is calcite. The concentration of suspended calcite changes significantly within the water column. Giovanoli et at. (1980) observed a relatively constant concentration of calcite in suspended matter collected at four depths (9, 82, 123, and 133 m) in August and September in Lake
Zurich. However, significant changes in concentration of particulate calcite could be an indicator of the trophic state of Lake Erie, Le., increased rate of photosynthesis and respiration, especially at the water surface. Morphology and Composition of Individual Particles
Micrographs selected for this publication were the most representative from about 300 investigated individual particles collected at the three sampling stations in Lake Erie. The investigation was carried out on particles previously separated into various size fractions on membrane filters during the sampling as outlined in Materials and Methods. Suspended matter collected from a I-m depth at all sampling stations during all sampling periods was composed mainly of particles and flocculates > 3 Ikm. On the other hand, suspended matter collected 1 m above the bottom included mainly individual particles < 3 Ikm with the exception of the sample obtained at the central basin (station 94) in September. The majority of particles in the latter sample consisted of flocculated particles of organic and inorganic origin. Samples collected in the middle of the water column at the deepest point contained mainly inorganic particles < 3 Itm. A mixture of flocculates and phytoplankton fragments occurred at this depth in September. With the exception of May, suspended solids from the 6-m depth at station 94 consisted mostly of large flocculates of organic material. The flocculation of inorganic and organic particles plays an important role in settling velocity of both components. It induces faster sinking of organic particles due to the higher specific gravity of associated minerals and, on the other hand, can keep the heavier mineral particles afloat. Particles >8 Ikm consisted mostly of various algal species, broken diatoms, and flocculates of organic material with smaller particles of inorganic origin. Figure 3 shows a micrograph and chemical composition of green algae of species Staurastrum Paradoxum, which are inhabitants of eutrophic waters. An EDX spectrum was obtained at the area indicated by an arrow. Detected elements (Si, S, CI, K, and Cal were considered to be cellular constituents. Flocculated particles are shown in Figures 4 and 5, the spectra of the first flocculate indicated by a high Si peak, the presence of a piece of diatom
LAKE ERIE SUSPENDED MATTER 1000
Ca K
100
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ENERGY (KEV)
FIG. 3. Scanning electron micrograph with the X-ray spectrum of the green algae Staurastrum paradoxum. " CI
2000
Si
1500 Pb
",
~
Z 1000 ::J
..
0
()
K
500
"
.'
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r • ",'
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. ~r;.;.;.'£~4"-.~ .. ~, .._~ ... ~
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0+------,------,-----.-------.---------, o 4 6 2 8 10 ENERGY (KEV)
FIG. 4. Scanning electron micrograph with the X-ray spectrum offlocculated particles collected at I-m water depth in Lake Erie.
293
294
A.MUDROCH
100K
s, :~
10 K
1000
~
Z
:::l
o()
100
10
0+-----,--------,----,-------,-------,
o
2
4 6 ENERGY (KEV)
8
10
FIG. 5. Scanning electron micrograph of a flocculated particle collected at I-m water depth showing the X-ray spectrum obtained from the area indicated by an arrow.
frustula. The absence of Al in this spectrum documents that alumino-silicates were absent and that CI, K, and Ca were constituents of organic material. Figure 5 shows the morphology of a number of particles < 8 /tm, including broken diatoms and other algal species. The EDX spectrum corresponds to one of the flocculates, which is most likely composed of particles of mineral origin as indicated by the presence of AI. The flocculate shown in Figure 6 is of inorganic nature as indicated by the presence of AI. This flocculate seems to be associated with some remnant of biological material, probably a membrane of algal cell showing on the micrograph as a "ghost." The flocculate contains a significant quantity of Fe. Presence of S could be associated with the remnants of the organic matter. Many particles of size < 3 /tm and > 8 /tm were of inorganic origin, i.e., mineral particles, or flocculates of smaller mineral particles. Two particles of this size shown in Figure 7 were morphologically similar, but their composition was different. The spectrum of particle no. 1 was similar to that of biotite (Mudroch et al. 1977). The other particle was identified as calcite. The Ca peak in the biotite
spectrum probably originated from the closely attached calcite particle. Figure 8 shows a number of particles of detrital origin of size 3 to 8 /tm. The elemental spectrum of particle No.1 was similar to that of vermiculite, possibly with K in the interlayer position. The spectrum of particle No.2 resembled one of the titanium minerals. The majority of the particles present in the < 3/tm size fraction were nonflocculated mineral fragments. Figure 9 shows morphology of a number of such particles and a spectrum of one of them. Minerals in this size fraction were illite, feldspars, muscovite, biotite, and calcite. The electron microscopy investigation supported the findings of mineralogical and chemical analyses. It showed that Fe was abundant in individual particles and flocculates of various sizes and origin and also occurred incorporated in mineral structures (biotite, vermiculite, muscovite, illite). Mn was found incorporated in some minerals (biotite, vermiculite) and as the major component of particles found at station 2 and identified as a manganese precipitating microorganism Metallogenium personatum (Mudroch and Bistricki 1981). Si was present in diatoms and other algal aluminosilicates
295
LAKE ERIE SUSPENDED MATTER
100K
10 K
Si .~
1000
.. ..
AI :'. ~
~
::
z
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::::>
o o
;...
,: :.
100
K
.,.S
:.;
!:
.
~
•
'#ti!., ~:.
~~
.it .
I
Fe
Ca
j
.'.
~
i':.. :'~. •:. ;:
~v~.
-:!~rti· .. :
. . . :.;,..
T"I
01.
:"
~'. ~ ~; ~~~~~ l:-,."~,_:. ~\.,.,~ ~'..~.' _~.; : _~ -
-:
0"
...0- .::.
'I
•
~ :-:~
Zn
'~:'~',f':- ~~~}i: .~.. :~.. -;:0.. :..:·,c~. }.•... ..... '" ' "."--- .-0 -::-:.:.0::.0:' . "'!.: ::0.: ..... .....
~
•
0-:'.:'.0
10
Ni
i
AI '.
•••••••• 0:'\wo.. ::_ ° 0 . . :..~
.-,.. • :~:.:
YO
•
:':_'
...
0--+------.,--------.---------.-----,----------,
o
2
4
6
8
10
ENERGY (KEV)
FIG. 6. Scanning electron micrograph offlocculated mineral particles associated with remnants of biological origin. X-ray spectrum was obtained from the area indicated by an arrow.
and silica grains. K was present in both organic matter and minerals; P, Sand CI were found mostly in particles of organic origin and Ti appeared to be associated with mineral particles. Ca occurred in particles of organic origin and particles of diverse morphology containing only Ca and identified as CaC0 3 • SUMMARY AND CONCLUSIONS Suspended matter was collected in Lake Erie in 1978 at the following localities and time: from 1m, 6-m, and 13-m water depth at a station in the central basin in May, August, and September; and from 1-m, 30-m and 61-m water depth at the deep-
est point of the lake in May and September. The water depth at each station was 14 m, 22 m, and 62 m, respectively. Mineralogical and chemical composition of the sampled suspended matter were investigated together with the morphology of individual particles. Investigation by electron microscopy supported findings of the geochemical and mineralogical examination and revealed morphological features and associations of particles of biological and inorganic origin. The analysed elements were partitioned between inorganic and biological material in the following way: AI, Ti, and Mg were associated only with inorganic material; Si, Fe, Ca, K, Mn, and P were associated with both biological and inorganic material. The majority of the deter-
296
A.MUDROCH
10 K
Si !'. AI ::
::
1000
'
. .. :.. ~
~
~
Ca
Fe ~
Z ::J
:':
.
o()
..,l
100
~
,;
.;-,;l;.. _V"~
:.
10
0+------,--------,--------,--------,-----------, o 4 6 10 2 8 ENERGY (KEV)
FIG. 7. Scanning electron micrograph of mineral fragments collected at 1 m above the lake bottom.
mined elements were present in more than one chemical form. Inorganic material was composed mainly of mineral fragments and biological material included pieces of diatoms, other algal species, and amorphous particles and flocculates of unknown origin. Flocculates > 3 J.tm composed of organic material and mineral fragments were common at the water surface and middle of the water column. Mineral fragments < 3 J.tm were the major constituents of suspended matter at the bottom. However, in late summer, large flocculates predominated near the bottom at the shallow sampling station. The major components of the suspended matter in Lake Erie were organic material, aluminosilicates, and calcite. The high levels of organic material and fluctuations of calcite concentration in suspended matter indicated a high rate of photo-
synthesis and respiration in the epilimnion during summer. The concentration of organic C in suspended matter exceeded many times that of the surface sediment, especially in August and September, indicating decomposition and transport of organic matter within the water column or at the sediment-water interface. ACKNOWLEDGMENTS
I would like to thank Dr. K. R. Lum for providing samples, Mr. T. Bistricki for the help with the electron microscopy investigation, Mr. W. Finn and his associates for the technical arrangement of the micrographs and spectra, and Drs. R. J. Allan and R. J. Maguire for their critical review of the manuscript.
LAKE ERIE SUSPENDED MATTER
297
100K
Si
:.~
10 K
AI .:
~:
Ca
:: "
:
::. Mg:
1000
~
!It
J
~
• . ,.=
Z
!
~
o
o
100
...,.,..
10
0-1-------,.-------.--------.------,---------, 10 8 4 6 o 2 ENERGY (KEV)
FIG. 8. Scanning electron micrograph of mineral fragments collected at 1 m above the lake bottom. The X-ray spectrum is representative for the three particles (No.1) indicated by arrows.
REFERENCES Burns, N. M. 1976. Lake Erie in the early seventies. J. Fish. Res. Board Can. 33, Special Issue. Camp, D. C., Van Lehn, A. L., Rhodes, J. R., and Pradzynski, A. H. 1975. Intercomparison of trace elements determinations in simulated and real air particulate samples. X-Ray Spectrometry 4: 123-137. Deuser, W. G., Brewer, P. G., Jickells, T. D., and Commeau, R. F. 1983. Biological control of the removal of abiogenic particles from the surface ocean. Science 219 (4583):388-291. Giovanoli, R., Briitsch, R., Diem, D., Osman-Sigg, G., and Sigg, L. 1980. The composition of settling particles in Lake Ziirich. Schweiz. Z. Hydro/. 42:89-100. Goldschmidt, V. M. 1958. Geochemistry. Ed. A. Muir, pp. 47-68. Oxford: Clarendon Press. Herdendorf, C. E., Hartley, S. M., and Charlesworth, L. J. 1974. Lake Erie bibliography in environmental sciences. Bull. Ohio Bio. Soc. 4:1-113. Holmes, G. S. 1981. The limitations of accurate "thinfilm" X-ray fluorescence analysis of natural particulate matter: problems and solutions. Chem. Geo/. 33:333-353.
Kemp, A. L. W., Thomas, R. L., Dell, C. I., and Jaquet, J .-M. 1976. Cultural impact on the geochemistry of sediments in Lake Erie. J. Fish. Res. Board Can. 33:440-462. ____ , Thomas, R. L., and Williams, J. D. H. 1977. Major elements, trace elements, sediment particle size, water content, Eh and pH in 26 cores from Lakes Superior, Huron, Erie and Ontario. Environment Canada, Canada Centre for Inland Waters, Burlington, Ontario. Unpublished Report. Krauskopf, K. B. 1967. Introduction to Geochemistry. New York: McGraw-Hill. Mudroch, A. 1977. Determination of major elements in lake and river bottom sediments by Philips P. W. 1220C X-ray fluorescence spectrometer. Environment Canada, Canada Centre for Inland Waters, Burlington, Ontario, Unpublished Report. ____ , and Bistricki, T. 1981. Occurrence of manganese-rich microparticles in the Eastern Basin of Lake Erie. Can. Miner. 19:435-440. ____ , Zeman, A. J., and Sandilands, R. 1977. Identification of mineral particles in fine grained lacustrine sediments with transmission electron
A.MUDROCH
298 100K
10 K
.
Si
1000
AI
~:
'. ::;l ..
g?
z
::::>
o
u
Fe
100
.'.
.....: .
~t
...
:£.: 10
O-t------,------,-----.--------.-----.,
o
2
4 6 ENERGY (KEV)
8
10
FIG. 9. Scanning electron micrograph of nonflocculated mineral fragments < 3 Jlm in size collected at a 61-m depth in Lake Erie. X-ray spectrum corresponds to the particle indicated by an arrow.
microscope and X-ray energy dispersive spectroscopy. J. Sed. Petrol. 47:244-250. Munawar, M., and Munawar, I. F. 1976. A lakewide study of phytoplankton biomass and its species composition in Lake Erie, April-December, 1970. J. Fish. Res. Board Can. 33:581-600. Murphy, T. P., Hall, K. J., and Yesaki, I. 1983. Coprecipitation of phosphate with calcite in a naturally eutrophic lake. Limnol. Oceanogr. 28:58-69. Nriagu, J. 0., Wong, H. K. T., and Coker, R. D. 1981. Particulate and dissolved trace metals in Lake Ontario. Water Research 15:91-96. Price, N. B., and Calvert, S. E. 1973. A study of the geochemistry of suspended particulate matter in coastal waters. Mar. Chem. 1:169-189. Rossknecht, V. H. 1980. Phosphate removal with calcium carbonate precipitation in the Lake of Constance (Obersee) (in German). Arch. Hydrobiol. 88:328-344. Rossmann, R. 1980. Relative importance of authigenic calcite to the chemistry of particulate matter in southeastern Lake Michigan. In Abstracts of the 23rd Conf. Great Lakes Res., InternaL Assoc. Great
Lakes Res. Sholkovitz, E. R. 1979. Chemical and physical processes controlling the chemical composition of suspended material in the River Tay estuary. Estuar. Coast. Mar. Sci. 8:523-545. Sozanski, A. G., and Cronan, D. S. 1979. Four manganese concentrations in Shebandowan Lakes, Ontario. Can. J. Earth Sci. 16:125-140. Spencer, D. W., and Sachs, P. L. 1970. Some aspects of the distribution, chemistry and mineralogy of suspended matter in the Gulf of Maine. Marine Oeol. 9:117-136. Strong, A. E., and Eadie, B. J. 1978. Satellite observations of calcium carbonate precipitations in the Great lakes. Limnol. Oceanogr. 23:877-887. Vollenweider, R. A., Munawar, M., and Stadelmann, P. 1974. A comparative review of phytoplankton and primary production in the Laurentian Great Lakes. J. Fish. Res. Board Can. 31:737-762. Wright, R. F., and Nydeggen, P. 1980. Sedimentation of detrital particulate matter in lakes: influence of currents produced by inflowing rivers. Water Res. 16:597-601.
CHEMISTRY, MINERALOGY, AND MORPHOLOGY OF LAKE ERIE SUSPENDED MATTER
A. Mudroch
Environmental Contaminants Division National Water Research Institute Burlington, Ontario L7R 4A6
ABSTRACT. Suspended matter was collected from different depths at three stations in spring and summer, 1978, in Lake Erie. Chemistry, mineralogy, and morphology of the suspended particles were measured to investigate spatial and temporal changes. The determined elements (Si, Fe, Ca, K, Mn, P, AI, Ti, and Mg) were partitioned between inorganic and biological material and the majority of these elements were present in at least two different chemical forms. Flocculates > 3 Jlm composed of organic material and mineral fragments were common at the water surface and the middle of the water column. Mineral fragments < 3 Jlm were the major constituent of suspended matter at the bottom at the deepest sampling station (62 m). The concentration of the major components of the suspended matter, organic material, alumino-silicates, and calcite, varied significantly from spring until later summer. The high concentration oforganic material and the fluctuations ofcalcite conce/l? tration result from high rates of photosynthesis and respiration and temperature increases in spring and summer. ADDITIONAL INDEX WORDS: Organic matter, electron microscopy, X-ray fluorescence.
(Vollenweider et al. 1974) with abundant suspended particles derived from shoreline erosion, river inputs, aeolian material, autochtonous organic matter, and bottom sediment resuspension. The objective of the present study was to characterize the suspended matter in the lake by mineralogical and chemical composition and by investigation of the morphology and associations of individual particles in the water column.
INTRODUCTION Suspended matter is an important component of any aquatic ecosystem for its ability to transport contaminants and nutrients. It provides a very large surface area available to various chemical reactions which may invalidate many of the dilute solution chemistry equilibria. Past studies on suspended matter in the marine environment (Price and Calvert 1973, Deuser et al. 1983), river estuaries (Sholkovitz 1979), and in freshwater systems (Wright and Nydeggen 1980, Giovanoli et al. 1980) included investigation of the distribution, flux, and sedimentation pattern of suspended matter and temporal and spatial changes in its chemical composition. The knowledge of the character and composition of particles suspended in the water column are a necessary step for further understanding of pathways and cycling of contaminants in a lake. Many investigations were carried out in the past on limnological processes within Lake Erie (Herdendorf et al. 1974, Burns 1976). This lake is known to be mesotrophic to eutrophic in nature
MATERIALS AND METHODS Suspended matter was collected during 1978 in Lake Erie at three stations: Nos. 2 and 94 in May, August, and September and at No. 23 in May and September (Fig. 1). These three stations were selected for their following features: station 2 was located in the eastern basin of Lake Erie at a water depth of 22 m. Station 94 at a water depth of 14 m was the shallowest of the three stations and located in the central basin, and station 23 was at the deepest point of Lake Erie (62 m). At each station about 600 L of lake water were 286
287
LAKE ERIE SUSPENDED MATTER 43"f-~~-+-~~~1r'='=----+-~~~~~+-~~~~~+-~~~------
Lake Erie 42°
•, (J
I?
83°
<:J
82°
81°
80°
79
FIG. 1. Sampling localities in Lake Erie.
pumped from a I-m depth, mid-water column, and I-m above the sediment-water interface into polyethylene containers and then centrifuged through a continuous-flow Westphalia separator. The collected suspended matter was then freeze-dried and homogenized in an agate mortar. Surface 0 to l-cm sediment was collected by subsampling a benthos core collected at each sampling station. The sediment was freeze-dried and ground to 189 p,m size (100 mesh) for further analyses. The quantity of suspended matter collected by centrifugation was less than 1 g. Consequently the X-ray fluorescence spectrometry technique described by Sholkovitz (1979), using deposited suspended matter on filter papers, was considered the only practical method for the quantitative determination of Si, AI, Fe, Mg, Ca, Na, K, Ti, and P. The problems involved in the X-ray fluorescence analyses of natural particulate matter deposited on a filter paper are mainly the grain size and the composition of individual particles. Holmes (1981) theoretically and experimentally examined the method and proposed several ways to deal with the problems, for example the use of standards and samples of a similar composition and grain size, or to correct for absorption and enhancement effects by equations containing relevant parameters. Camp et al. (1975) presented results of an interlaboratory study of element determination in simulated and real particulate samples. Thin layers of similar mass (200 p,g cm- Z) were prepared by resuspending the dry homogenized samples in 250 mL of distilled water and filtering
identical aliquots onto 0.45-p,m Nuclepore membrane filters. Geological reference samples used as standards were prepared by the same method. A Philips X-ray fluorescence spectrometer PW 1450 was programmed for the determination of major elements (Cr target, 50 kV/50mA, argon-methane gas, flow proportional gas counter, PET, TLAP, AND, and LIF crystals). Four samples were run simultaneously: the first sample was a drift monitor powder pellet, the second sample was a blank Nuclepore filter, and the others were duplicates of the same sample. Both calibration samples remained constantly in the analyzer. A computer program based on two standard methods using six geological standards was used for the calculation of major elements in samples. Precision of the analyses was determined by analyzing, in quadruplicate, major elements in six various geological standards and one lake bottom sediment sample obtained from station no. 2 in Lake Erie (Fig. 1). The Q- test was carried out to determine whether at the 900;0 confidence level any results should be rejected. All results were retained. Relative standard deviations were in the following range: SiOz 1-3%, Al z0 3 4-6%, Fe z0 3 3-10%, MgO 4-14%, CaO 4-8%, NazO 5-15%, KzO 3-8%, TiOz 3-10%, MnO 10-20%, and PzOs 5-20%. The limit of detection and the limit of quantitation were defined to determine if the measured values of samples on Nuclepore filters (SI) were significantly different from those found for blank Nuclepore filters (Sb)' With the exception of NazO (SI - Sb = 3a) and MnO (SI - Sb = 8a),
288
A.MUDROCH
Le., standard deviations in the region of less certain quantitation, the values for all elements were greater than lOa, Le., the level above which quantitative results may be obtained with a specific degree of confidence. To determine the accuracy of the applied X-ray fluorescence technique two standards (granite and syenite) were deposited on Nuclepore filters in duplicate and analyzed by the described method. The relative errors of the calculated concentration of both analyzed standards were as follows: SiOz 2070, Alz0 3 4070, Fe z0 3 5070, MgO 14070, CaO 6070, NazO 20070, KzO 8070, TiOz 10070, MnO 25070, and P Z05 20070. The accuracy evaluation should be interpreted bearing in mind that geological standard reference materials do not fully simulate natural samples such as bottom sediments and suspended matter. Qualitative determination of mineralogical composition of suspended matter was carried out by the powder X-ray diffraction technique using CuKa radiation. Analyses of surface sediment were carried out by X-ray fluorescence spectrometry using powder pellets (Mudroch 1977). Organic C was determined in suspended matter and surface sediments with a Leco carbo analyser. The investigation of morphology and chemical composition of individual particles in these samples was carried out with an AMR 1000 electron microscope (EM) equipped with an energy dispersive X-ray (EDX) spectrometer and a computerbased data processing system (Tracor-Northern, TN-ll). Identification of minerals from the EDX spectra was accomplished using the method of Mudroch et al. (1977). For the EM study about 100 mL of water from each sampling station and depth were filtered sequentially through 8-Jtm, 3-Jtm, and 0.45-Jtm pore size Unipore polycarbonate membranes. The membranes with deposited particles were dried in a dessicator, cut to appropriate size, and mounted on a specimen support stub using two-sided adhesive tape. To make the specimen surface thermally and electrically conductive, all prepared samples were covered by a 20-nm-thick layer of carbon. The carbon coating was performed in a vacuum evaporator at a pressure of 10- 5 torr. RESULTS AND DISCUSSION The mineralogy of the suspended matter showed a close similarity to that of bottom sediments (Kemp et al. 1976). X-ray diffraction revealed the pres-
ence of quartz, feldspars, calcite, illite, and small quantitites of chlorite, kaolinite, and dolomite. However, the quantities of each mineral at all three stations varied with sampling depth and time. Results of the quantitative determination of Si, AI, Fe, Mg, Ca, Na, K, Ti, Mn, P, and organic C in suspended matter and 0 to 1 cm surface sediment are presented in Table 1 as Jtg g - I dry matter. The concentration of organic C varied with depth and among the stations (Table 1) and diluted the inorganic components represented mainly by mineral particles. A gradual increase in organic C occurred in suspended matter near the bottom at stations 94 and 2 from May to September reflecting sedimentation of dead phytoplankton. The concentration of organic C was similar at all three sampling depths at the shallowest station (no. 94) in September due to the mixing of the entire water column. The organic C concentration was almost four times higher in suspended solids than in the surface sediment at this station in September, indicating decomposition and transport of organic matter in the water column or at the immediate sediment-water interface. At the deepest point of the lake (station 23) organic C was higher at the surface in May than in September, probably reflecting higher primary production at this area in spring than in later summer (Munawar and Munawar 1976). A significant decrease in organic C occurred toward the bottom at this station in both May and September. Similar concentrations of organic carbon were observed at a 30-m depth and at the bottom in May and September, indicating steady conditions below the thermocline during both months. The concentration of Al in suspended matter was almost two times higher at the bottom than at the surface at station 94 in May (Table 1). However, very little spatial variation was observed in Al at the same station in August and September. This station, located in the central basin, was the shallowest of all three sampling stations (water depth 14 m) and was not stratified in September. In May a greater than twofold increase in Al occurred toward the bottom at station 2 located in the eastern basin (water depth 22 m). This increase was still significant in August and September when the thermocline was about 3 m above the bottom. A more than twofold increase in Al was observed toward the bottom at station 23 (water depth 62 m) in samples collected in May and September. The concentration of Al in suspended matter was higher at the bottom at this station in September
289
LAKE ERIE SUSPENDED MATTER TABLE 1. Lake Erie, Suspended solids analyses station no. 2, Depth 22 m.
21 m
10m
1m
21 m
10m
1m
21 m
10m
1m
Bottom Sediment o to 1 cm (650/0 clay, 350/0 silt)
9.55
11.20
25.50
12.55
14.75
34.55
15.85
23.52
29.81
4.43
42.8 8.1 3.0 0.9 9.8 1.7 0.32 0.07 1.4
33.2 6.0 2.1 0.6 7.2 1.3 0.24 0.06 1.0
36.2 4.9 1.6 0.6 4.3 1.1 0.20 0.06 0.5
53.37 13.63 7.08 2.18 1.49 3.81 0.54 0.20 0.44
May Percent Organic C
September
August
Si02 53.1 45.4 24.9 41.1 39.9 28.5 A1 20 3 9.1 3.5 6.9 6.8 4.7 9.4 Fe20 3 3.8 2.4 1.4 2.9 2.3 1.6 MgO 1.0 0.5 0.6 0.6 0.6 0.7 8.1 6.7 CaO 8.4 9.8 5.3 9.6 2.4 K20 1.8 1.1 1.6 1.5 1.2 Ti0 2 0.36 0.34 0.14 0.28 0.27 0.16 MnO 0.09 0.08 0.05 0.08 0.06 0.05 P 20 S 0.9 0.8 1.4 0.9 1.0 1.3 Lake Erie, Suspended solids analyses station no. 23, Depth 62 m. May Percent
61 m
30 m
1m
61 m
30 m
1m
Bottom Sediment o to I cm (68% clay, 32% silt)
5.21
10.90
21.65
4.24
11.70
17.95
3.30
53.6 9.0 3.6 1.1 8.2 2.5 0.38 0.09 0.8
43.5 8.9 2.5 0.4 8.3 1.7 0.32 0.08 0.7
20.1 4.9 1.7 1.0 19.7 1.4 0.15 0.06 1.8
53.2 13.0 4.1 0.6 15.3 2.9 0.48 0.11 1.8
38.5 11.0 3.1 0.5 6.5 2.3 0.39 0.07 0.05
32.4 5.4 2.0 0.4 7.6 1.3 0.22 0.06 0.7
56.79 12.58 7.59 3.23 2.11 3.70 0.52 0.27 0.32
Organic C Si02 A1 20 s
Fe20 3 MgO CaO K20 Ti0 2 MnO P 20 S
September
Lake Erie, Suspended solids analyses station no. 94, Depth 14 m. May Percent Organic C Si02 A1 20 3 Fe203 MgO CaO Na20 K20 Ti0 2 MnO P 20 S
August
13m
6m
1m
13 m
6m
1m
13m
6m
1m
Bottom Sediment o to 1 cm (77% clay, 23% silt)
6.64
11.48
17.45
16.88
20.52
21.89
20.29
20.90
22.20
5.13
51.6 10.4 3.0 0.4 7.0 0.6 2.0 0.35 0.09 0.5
46.2 9.3 2.5 0.4 7.8 0.6 2.0 0.32 0.08 0.6
26.3 4.0 1.4 0.2 13.2 1.3 1.2 0.23 0.05 0.9
41.2 6.6 2.3 0.3 3.6 0.2 1.4 0.23 0.07 0.6
38.3 4.3 1.5 0.2 3.1 0.2 0.9 0.16 0.05 0.4
34.3 5.3 2.4 0.3 6.6 0.3 1.3 0.24 0.06 0.9
33.4 5.2 2.0 0.3 6.6 0.3 1.3 0.20 0.06 1.0
34.7 5.3 1.9 0.4 7.6 0.3 1.3 0.19 0.06 0.9
39.4 5.1 1.8 0.2 3.7 0.2 1.1 0.18 0.06 0.4
55.52 15.39 7.54 2.13 1.40 0.99 3.50 0.57 0.13 0.4
than in May and was similar to the Al concentration in the bottom sediment (Table 1). Spencer and Sachs (1970) have shown that Al in the particulate matter from the Gulf of Maine was almost entirely associated with alumino-silicates.
September
Our electron microscopy investigation showed that Al occurred exclusively in alumino-silicates in suspended matter in Lake Erie. The Al concentration therefore represents the quantity of suspended alumino-silicates.
290
A. MUDROCH
TABLE 2. Element ratios in Lake Erie suspended matter.
Station 2
Si/AI
Ca/Al
K/AI
Fe/AI
Mg/Al
Ti/Al
May
1m 10m 21 m
7.05 4.99 5.66
2.77 0.92 0.87
0.33 0.20 0.26
0.40 0.27 0.41
0.18 0.06 0.11
0.04 0.03 0.04
August
1m 10m 21 m
6.06 5.87 5.90
2.05 0.99 0.76
0.26 0.22 0.23
0.36 0.34 0.42
0.15 0.09 0.10
0.03 0.04 0.04
September
1m 10m 21 m
6.32 5.48 5.29
0.89 1.20 1.22
0.23 0.22 0.22
0.34 0.35 0.38
0.13 0.10 0.12
0.04 0.04 0.04
Sediment
3.84
0.11
0.28
0.52
0.16
0.04
Station 23
SiiAl
Ca/AI
K/Al
Fe/AI
Mg/AI
TiiAI
May
1m 30 m 61 m
4.04 4.97 5.92
3.96 0.92 0.91
0.30 0.20 0.27
0.35 0.28 0.41
0.20 0.06 0.12
0.03 0.04 0.04
September
1m 30 m 61 m
5.94 3.48 4.07
1.39 0.58 1.12
0.24 0.22 0.22
0.37 0.29 0.32
0.08 0.05 0.05
0.04 0.04 0.04
Sediment
4.51
0.17
0.29
0.60
0.26
0.04
Station 94
SiiAI
Ca/AI
K/AI
Fe/Ai
Mg/AI
Ti/AI
May
1m 6m 13m
6.52 4.95 4.93
3.28 0.84 0.67
0.30 0.22 0.20
0.36 0.27 0.29
0.05 0.04 0.04
0.06 0.03 0.03
August
1m 6m 13 m
6.48 8.77 6.19
1.24 0.73 0.55
0.25 0.22 0.22
0.47 0.35 0.35
0.07 0.07 0.05
0.05 0.04 0.03
September
1m 6m 13m
7.69 6.46 6.37
0.73 1.43 1.26
0.23 0.24 0.25
0.35 0.35 0.39
0.05 0.08 0.07
0.04 0.04 0.04
3.60
0.09
0.22
0.48
0.14
0.04
Sediment
Mineralogical investigation by X-ray diffraction showed that Si was present in suspended matter in alumino-silicates and quartz. The mean Si/Al ratio of shales is about 3.0 (Krauskopf 1967). The SilAl ratios were 3.84, 4.51, and 3.60 in the surface sediment at stations 2, 23, and 94, respectively (Table 2). The bottom sediment usually contains more detrital quartz relative to alumino-silicates than the suspended matter (Price and Calvert 1973). Therefore, the latter should have a lower Sil Al ratio. This was observed at station 23 at the surface in May and at the bottom and the middle of the water column in September. However, the
SilAl ratio was higher in the rest of suspended matter samples collected at station 23 and in all samples collected at stations 94 and 2 than in the surface sediment (Table 2). Electron microscopy showed that the excess of Si was of biogenic origin appearing mostly in diatom frustules. Biogenic Si concentrations were highest at the surface at stations 93 and 2 in May and at station 23 in September, reflecting seasonal change in biological production and species composition at each sampling locality (Munawar and Munawar 1976). The electron microscopy, X-ray fluorescence, and X-ray diffraction analyses indicate the presence of three
291
LAKE ERIE SUSPENDED MATTER
different forms of Si in suspended matter: biogenic, and that as quartz and alumino-silicates. The Ca concentration and Cal Al ratio are shown in Tables 1 and 2, respectively. According to Goldschmidt (1958), a carbonate-free, finegrained sediment has a Cal Al ratio of 0.033. This value was exceeded many times in suspended solids collected in Lake Erie, because the soils in the drainage basin are mainly derived from sedimentary rocks where calcite is a common mineral. The Cal Al ratio in the sediment was 0.11, 0.17, and 0.09 at stations 2, 23, and 94, respectively. The ratio was much higher in suspended matter, particularly at the I-m depth at all three sampling stations in May. Mineralogical analyses showed that the majority of Ca in all samples was present as calcite precipitated in the lake or as detrital grains introduced from the drainage basin. Precipitation of CaC0 3 is a significant process in the Great Lakes. Strong and Eadie (1978) have shown that precipitation is greatest during periods of thermal stratification, and Rossmann (1980) found that the composition of the suspended solids in southeastern Lake Michigan is dominated by authigenic calcite in August and September. The temporal increase in Ca concentration in suspended matter indicated the presence of precipitated CaC0 3 • Precipitation at the three sampling stations was highest at the surface during May when the surface water temperature started to increase. In September the CalAl ratio increased at the bottom, most likely showing the sedimentation of precipitated CaC0 3 • The variations of the Cal Al ratio between the stations in August and September may reflect the changes in pH introduced by the primary producers followed by calcite precipitation. High correlations of Al with K and Ti in suspended matter indicated association of these two elements with alumino-silicates (Fig. 2). The X-ray diffraction and EM investigation showed that the alumino-silicate particles were mostly clay minerals, biotite, and feldspars. The KI Al ratio in suspended matter was similar to that found in the sediment at stations 2 and 23 (Table 2). Krauskopf (1967) gave a KI Al ratio of 0.29 for an average shale. The EM investigation indicated that particles > 2 JLm had a higher feldspar I clay ratio compared to the < 2-JLm particles, as well as a higher KI Al ratio. Variable quantities of feldspars in suspended matter most likely contributed to the small variations in the KI Al ratio at each station. Analyses of particles of biological origin by EDX showed that some K was also incorporated in algal cells
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and may affect the KI Al ratio, especially in suspended matter at the surface. A fraction of Mg was held in clay minerals. However, no significant correlation was found between Mg and Al because Mg was partly present in dolomite particles of detrital origin. According to Krauskopf (1967), the average shale MgI Al ratio is 0.17, which is close to that found in the Lake Erie fine-grained sediments (majority of < 2 JLm size particles). Poor correlation between Al and Fe in suspended matter (correlation coefficient = 0.145, no. samples 24) suggested that the occurrence of suspended Fe is not controlled by alumino-silicates and that Fe is present in other forms. The Fe was found in almost every individual particle of organic or inorganic origin examined by EMI EDX. The Fel Al ratio in 0- to l-cm surface sediment was almost two times higher than in suspended matter and reflected the migration of Fe in the sediment followed by precipitation at the sediment-water interface. Data of Kemp et al. (1977) show a significant decrease of the Fel Al ratio in sediment below the 0 to l-cm layer. Manganese behaved similarly to Fe, indicating little incorporation in alumino-silicates. Manganese concentration was higher in the surface sedi-
292
A.MUDROCH
ment at each sampling station than that in suspended matter reflecting similar migration and precipitation as Fe (Sozanski and Cronan 1979). The TilAl ratio was similar in suspended matter and sediment at each sampling station (Table 2). The EM investigation showed that Ti was present mainly in alumino-silicate fragments. The concentration of particulate P was highest at the surface at all three sampling stations in May (Table 1). It remained highest at the surface at stations 94 and 2 in August; however, a significant decrease of P was observed at the surface followed by an increase at the bottom at all three sampling stations in September. No relationship was found between organic C and P, and between Fe and P. Significant correlation existed between Ca and P concentration (correlation coefficient = 0.812, no. samples 24), suggesting a possible coprecipitation of P with calcite similar to that observed in other studies (Rossknecht 1980, Murphy et at. 1983). However, seasonal variations in P concentration also indicated association with organic material. The P concentration in suspended matter was up to three times higher than in the surface sediment, indicating regeneration of P within the water column. The results show that organic material is one of the major components of suspended matter in Lake Erie. The phytoplankton bloom is obviously responsible for the high levels of organic matter in the epilimnion compared to the hypolimnion of the deep water in eastern Lake Erie. The results further indicate that the concentration of organic matter in the epilimnion increases with decreasing distance from the shore. Nriagu et at. (1981) suggested that organic matter represents an important vehicle for metal transport to lake sediment. However, after decomposition of organic matter in the water column the associated metals may become chemically and biologically available. On the other hand, metals will most likely become associated with sediment particles after decomposition of organic matter at the sediment-water interface. Consequently, the decomposition and settling rates of organic matter are important pathways of the metals cycling in the lake. Another important component of suspended matter is calcite. The concentration of suspended calcite changes significantly within the water column. Giovanoli et at. (1980) observed a relatively constant concentration of calcite in suspended matter collected at four depths (9, 82, 123, and 133 m) in August and September in Lake
Zurich. However, significant changes in concentration of particulate calcite could be an indicator of the trophic state of Lake Erie, Le., increased rate of photosynthesis and respiration, especially at the water surface. Morphology and Composition of Individual Particles
Micrographs selected for this publication were the most representative from about 300 investigated individual particles collected at the three sampling stations in Lake Erie. The investigation was carried out on particles previously separated into various size fractions on membrane filters during the sampling as outlined in Materials and Methods. Suspended matter collected from a I-m depth at all sampling stations during all sampling periods was composed mainly of particles and flocculates > 3 Ikm. On the other hand, suspended matter collected 1 m above the bottom included mainly individual particles < 3 Ikm with the exception of the sample obtained at the central basin (station 94) in September. The majority of particles in the latter sample consisted of flocculated particles of organic and inorganic origin. Samples collected in the middle of the water column at the deepest point contained mainly inorganic particles < 3 Itm. A mixture of flocculates and phytoplankton fragments occurred at this depth in September. With the exception of May, suspended solids from the 6-m depth at station 94 consisted mostly of large flocculates of organic material. The flocculation of inorganic and organic particles plays an important role in settling velocity of both components. It induces faster sinking of organic particles due to the higher specific gravity of associated minerals and, on the other hand, can keep the heavier mineral particles afloat. Particles >8 Ikm consisted mostly of various algal species, broken diatoms, and flocculates of organic material with smaller particles of inorganic origin. Figure 3 shows a micrograph and chemical composition of green algae of species Staurastrum Paradoxum, which are inhabitants of eutrophic waters. An EDX spectrum was obtained at the area indicated by an arrow. Detected elements (Si, S, CI, K, and Cal were considered to be cellular constituents. Flocculated particles are shown in Figures 4 and 5, the spectra of the first flocculate indicated by a high Si peak, the presence of a piece of diatom
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frustula. The absence of Al in this spectrum documents that alumino-silicates were absent and that CI, K, and Ca were constituents of organic material. Figure 5 shows the morphology of a number of particles < 8 /tm, including broken diatoms and other algal species. The EDX spectrum corresponds to one of the flocculates, which is most likely composed of particles of mineral origin as indicated by the presence of AI. The flocculate shown in Figure 6 is of inorganic nature as indicated by the presence of AI. This flocculate seems to be associated with some remnant of biological material, probably a membrane of algal cell showing on the micrograph as a "ghost." The flocculate contains a significant quantity of Fe. Presence of S could be associated with the remnants of the organic matter. Many particles of size < 3 /tm and > 8 /tm were of inorganic origin, i.e., mineral particles, or flocculates of smaller mineral particles. Two particles of this size shown in Figure 7 were morphologically similar, but their composition was different. The spectrum of particle no. 1 was similar to that of biotite (Mudroch et al. 1977). The other particle was identified as calcite. The Ca peak in the biotite
spectrum probably originated from the closely attached calcite particle. Figure 8 shows a number of particles of detrital origin of size 3 to 8 /tm. The elemental spectrum of particle No.1 was similar to that of vermiculite, possibly with K in the interlayer position. The spectrum of particle No.2 resembled one of the titanium minerals. The majority of the particles present in the < 3/tm size fraction were nonflocculated mineral fragments. Figure 9 shows morphology of a number of such particles and a spectrum of one of them. Minerals in this size fraction were illite, feldspars, muscovite, biotite, and calcite. The electron microscopy investigation supported the findings of mineralogical and chemical analyses. It showed that Fe was abundant in individual particles and flocculates of various sizes and origin and also occurred incorporated in mineral structures (biotite, vermiculite, muscovite, illite). Mn was found incorporated in some minerals (biotite, vermiculite) and as the major component of particles found at station 2 and identified as a manganese precipitating microorganism Metallogenium personatum (Mudroch and Bistricki 1981). Si was present in diatoms and other algal aluminosilicates
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and silica grains. K was present in both organic matter and minerals; P, Sand CI were found mostly in particles of organic origin and Ti appeared to be associated with mineral particles. Ca occurred in particles of organic origin and particles of diverse morphology containing only Ca and identified as CaC0 3 • SUMMARY AND CONCLUSIONS Suspended matter was collected in Lake Erie in 1978 at the following localities and time: from 1m, 6-m, and 13-m water depth at a station in the central basin in May, August, and September; and from 1-m, 30-m and 61-m water depth at the deep-
est point of the lake in May and September. The water depth at each station was 14 m, 22 m, and 62 m, respectively. Mineralogical and chemical composition of the sampled suspended matter were investigated together with the morphology of individual particles. Investigation by electron microscopy supported findings of the geochemical and mineralogical examination and revealed morphological features and associations of particles of biological and inorganic origin. The analysed elements were partitioned between inorganic and biological material in the following way: AI, Ti, and Mg were associated only with inorganic material; Si, Fe, Ca, K, Mn, and P were associated with both biological and inorganic material. The majority of the deter-
296
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mined elements were present in more than one chemical form. Inorganic material was composed mainly of mineral fragments and biological material included pieces of diatoms, other algal species, and amorphous particles and flocculates of unknown origin. Flocculates > 3 J.tm composed of organic material and mineral fragments were common at the water surface and middle of the water column. Mineral fragments < 3 J.tm were the major constituents of suspended matter at the bottom. However, in late summer, large flocculates predominated near the bottom at the shallow sampling station. The major components of the suspended matter in Lake Erie were organic material, aluminosilicates, and calcite. The high levels of organic material and fluctuations of calcite concentration in suspended matter indicated a high rate of photo-
synthesis and respiration in the epilimnion during summer. The concentration of organic C in suspended matter exceeded many times that of the surface sediment, especially in August and September, indicating decomposition and transport of organic matter within the water column or at the sediment-water interface. ACKNOWLEDGMENTS
I would like to thank Dr. K. R. Lum for providing samples, Mr. T. Bistricki for the help with the electron microscopy investigation, Mr. W. Finn and his associates for the technical arrangement of the micrographs and spectra, and Drs. R. J. Allan and R. J. Maguire for their critical review of the manuscript.
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REFERENCES Burns, N. M. 1976. Lake Erie in the early seventies. J. Fish. Res. Board Can. 33, Special Issue. Camp, D. C., Van Lehn, A. L., Rhodes, J. R., and Pradzynski, A. H. 1975. Intercomparison of trace elements determinations in simulated and real air particulate samples. X-Ray Spectrometry 4: 123-137. Deuser, W. G., Brewer, P. G., Jickells, T. D., and Commeau, R. F. 1983. Biological control of the removal of abiogenic particles from the surface ocean. Science 219 (4583):388-291. Giovanoli, R., Briitsch, R., Diem, D., Osman-Sigg, G., and Sigg, L. 1980. The composition of settling particles in Lake Ziirich. Schweiz. Z. Hydro/. 42:89-100. Goldschmidt, V. M. 1958. Geochemistry. Ed. A. Muir, pp. 47-68. Oxford: Clarendon Press. Herdendorf, C. E., Hartley, S. M., and Charlesworth, L. J. 1974. Lake Erie bibliography in environmental sciences. Bull. Ohio Bio. Soc. 4:1-113. Holmes, G. S. 1981. The limitations of accurate "thinfilm" X-ray fluorescence analysis of natural particulate matter: problems and solutions. Chem. Geo/. 33:333-353.
Kemp, A. L. W., Thomas, R. L., Dell, C. I., and Jaquet, J .-M. 1976. Cultural impact on the geochemistry of sediments in Lake Erie. J. Fish. Res. Board Can. 33:440-462. ____ , Thomas, R. L., and Williams, J. D. H. 1977. Major elements, trace elements, sediment particle size, water content, Eh and pH in 26 cores from Lakes Superior, Huron, Erie and Ontario. Environment Canada, Canada Centre for Inland Waters, Burlington, Ontario. Unpublished Report. Krauskopf, K. B. 1967. Introduction to Geochemistry. New York: McGraw-Hill. Mudroch, A. 1977. Determination of major elements in lake and river bottom sediments by Philips P. W. 1220C X-ray fluorescence spectrometer. Environment Canada, Canada Centre for Inland Waters, Burlington, Ontario, Unpublished Report. ____ , and Bistricki, T. 1981. Occurrence of manganese-rich microparticles in the Eastern Basin of Lake Erie. Can. Miner. 19:435-440. ____ , Zeman, A. J., and Sandilands, R. 1977. Identification of mineral particles in fine grained lacustrine sediments with transmission electron
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298 100K
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FIG. 9. Scanning electron micrograph of nonflocculated mineral fragments < 3 Jlm in size collected at a 61-m depth in Lake Erie. X-ray spectrum corresponds to the particle indicated by an arrow.
microscope and X-ray energy dispersive spectroscopy. J. Sed. Petrol. 47:244-250. Munawar, M., and Munawar, I. F. 1976. A lakewide study of phytoplankton biomass and its species composition in Lake Erie, April-December, 1970. J. Fish. Res. Board Can. 33:581-600. Murphy, T. P., Hall, K. J., and Yesaki, I. 1983. Coprecipitation of phosphate with calcite in a naturally eutrophic lake. Limnol. Oceanogr. 28:58-69. Nriagu, J. 0., Wong, H. K. T., and Coker, R. D. 1981. Particulate and dissolved trace metals in Lake Ontario. Water Research 15:91-96. Price, N. B., and Calvert, S. E. 1973. A study of the geochemistry of suspended particulate matter in coastal waters. Mar. Chem. 1:169-189. Rossknecht, V. H. 1980. Phosphate removal with calcium carbonate precipitation in the Lake of Constance (Obersee) (in German). Arch. Hydrobiol. 88:328-344. Rossmann, R. 1980. Relative importance of authigenic calcite to the chemistry of particulate matter in southeastern Lake Michigan. In Abstracts of the 23rd Conf. Great Lakes Res., InternaL Assoc. Great
Lakes Res. Sholkovitz, E. R. 1979. Chemical and physical processes controlling the chemical composition of suspended material in the River Tay estuary. Estuar. Coast. Mar. Sci. 8:523-545. Sozanski, A. G., and Cronan, D. S. 1979. Four manganese concentrations in Shebandowan Lakes, Ontario. Can. J. Earth Sci. 16:125-140. Spencer, D. W., and Sachs, P. L. 1970. Some aspects of the distribution, chemistry and mineralogy of suspended matter in the Gulf of Maine. Marine Oeol. 9:117-136. Strong, A. E., and Eadie, B. J. 1978. Satellite observations of calcium carbonate precipitations in the Great lakes. Limnol. Oceanogr. 23:877-887. Vollenweider, R. A., Munawar, M., and Stadelmann, P. 1974. A comparative review of phytoplankton and primary production in the Laurentian Great Lakes. J. Fish. Res. Board Can. 31:737-762. Wright, R. F., and Nydeggen, P. 1980. Sedimentation of detrital particulate matter in lakes: influence of currents produced by inflowing rivers. Water Res. 16:597-601.