On the relationship between lake trophic level and lake sediments

Water Res Vol. 18, No. 3. pp. 303-3t4. 1984 Printed in Great Britain. All rights r~erved

0043-1354 52 53 00-.-0.00 Copyright ~ 1984 Pergamon Press ktd

ON THE RELATIONSHIP BETWEEN LAKE TROPHIC LEVEL AND LAKE SEDIMENTS LARS H,I, KANSON

National Swedish Environment Protection Board, Water Laborator). Box 8043. S-75008 Uppsala. Sweden (Received December 1982)

Abstract--The purpose of this work has been to study the relationship between lake type. as expressed by various trophic characteristics, and sediment type, as expressed by determinations of simple chemical data on nitrogen, phosphorus, carbon and loss on ignition. The study is based on two sets of data. The first one emanates from 71 lakes for which information is available from published sources on trophic level and N-content and organic content (loss on ignition) from surficial sediments. More detailed information has been obtained from 12 Swedish lakes/basins on the following water indicators of trophic level: transparency, chlorophyll-a, total-N, totaI-P, organic-N, inorganic-N. The sediment data from these 12 lakes include: mean values and characteristic values for organic content (loss on ignition. IG) C-. Nand P-contents from surficial sediments. A BPN-value (bioproduction number) is defined by the slope coefficient of the regression line between N-content and IG-content of surficial sediments (0-1 cm). It has been shown that the BPN-value provides most accurate information about lake trophic level on the scale from oligotrophy to eutrophy0 provided that more than 50% of the lake area has lower loss on ignition than 20~/o (per dry substance). The BPN-value cannot be used as an indicator of trophic level if the lG-content is predominantly higher than 30% in a lake. In the latter case the ratio IG/N can be used as a means to differentiate lake humic level. The BPN-value, which is determined from sediment samples providing an even spread throughout the lake surface, yields lake typical information, whereas C/N rations yield site specific information. A key diagrammatic interpretation between lake trophic type and lake sediments has been presented. Key words--lake sediments, trophic status, nutrient elements

INTRODUCTION AND AIM OF THE WORK

Lake sediments can be regarded as a bank of environmental information. Most natural and/or anthropogenic activities in a lake and its drainage area will, directly or indirectly, leave an imprint in the sediments. Consequently, lake sediments are of interest in many limnological studies as well as in ecotoxicology and in aquatic pollution control programs (F6rstner and Wittmann, 1979; Hfi.kanson, 1981a). Welch (1980) states " . . . s e d i m e n t data used to indicate trophic change are P, N, and C content; however, no quantitative limits for these variables have been proposed". One aim of this paper is to propose such quantitative data, as mentioned by Welch, and to give a brief discussion on underlying causal relationships. "Lake sediments are the product of lake life. Consequently, they reflect the lake type". This quotation emanates from the old maestro of lake sedimentological studies in Sweden, G6sta Lundqvist, who made several major investigations on the relationships between lake type, sediment type and lake surroundings during the thirties and forties (Lundqvist, 1938, 1942). Another motive of this paper is to discuss lake type-sediment type relationships. Lakes can be classified according to several principles, e.g. based on their genetic/geological origin (Hutchinson, 1957), trophic level ( N a u m a n n , 1919, 1932: Thienemann, 1925, 1931; Rodhe, 1958, 1969 or 303 WR

1~3~D

Wetzel, 1975) or thermal regime (Hutchinson and Lfffler, 1956). Sediments can also be distinguished from many premises: from genetic/geological viewpoints (Norrman and K6nigsson, 1972), geochemical considerations (Berner, 1981) and according to numerous descriptive approaches from sediment characteristics like color, texture, structure, grain size, organic content, algal content or benthic community (Hansen, 1959; Stockner, 1972; Thomas et al., 1976; Saether, 1979). Therefore, when comparing lake type and sediment type, it is important to define the presuppositions. It should, at the outset, be noted that this is an interesting and important field of scientific endeavor, which is not very thoroughly or systematically explored at present. The aim of this work is not to try to synthesize the incongruous and scattered data from as many lake types and their sediments as possible into a new scheme that highlights probable but hitherto hidden links between the water and its "'tachometer", as the sediments may be called. Instead, the aim is to emphasize the principles for one such search for correlations, and to do this for the lake type as defined by the trophic level and the sediment type, as defined by certain readily accessible data on the nutrient elements (P, N and C, see Hansen, 1959, 1961; Tadajewski, 1966; Horie, 1969; Kemp, 1971; Premazzi and Ravera, 1977; Ravera and Parise, 1978). It should be stressed that no biological sediment parameters, like diatom frustules (Stock-

31)~

LAKS H.~KANSON Table I. Data on nitrogen (N!: carbon (C) and loss on ignition (IG) in various media N Varved clay Igneous rock Soils Fresh water Land animals Land plants Water vegetation (Fontinalis) Lake Sand, Sand, Silt', Silt*,

~'1 r, dS)

C ('~Ods)

C, N

0.08 0.02 0.1 0.23-10 -~ 10 3 3.2

0.55 0.20 2 11,10 -~ 46.5 45.4 29,8

6.9 I0 20 48 4.7 15 9.3

Kfgler and Larsen (19791 Bowen (19661 Bowen (19661 Bowen (1966) Bowen (1966) Bowen (19661 Hansen (1956)

N 0.3 0.4 0.6 0.7

IG 1.8 2.2 3.2 3.3

C/N 20 18 18 16

H~kanson(1977b) H:Ikanson(1977b) Htkanson(1977b) H/tkanson(1977b)

sediments: Lake V/inern Lake V/ittern Lake V~inern Lake V/ittern

Reference

*Post-glacial and pre-industrial silt.

net. 19721 or bottom fauna (Wiederholdm, 1979) will be discussed in this context. This work is entirely focused on standard chemical sediment variables. But among these there exist a great variety: inorganic-N, organic-N, total-N, apatite-P, organic-P, total-P, sulfides, carbonates, humic substances, fulvic substances, Fe/Mn-oxides and hydroxides, detritus, inorgamc biogenic matter, silicates, chlorophyll, loss on ignition, etc. These are parameters that may be linked, in some way, to lake trophic level, and it is probable that the best result would be obtained if several such parameters were used, thereby getting a more balanced index of general validity. The objective here, however, ~s not to scrutinize these possibilities. Instead. the aim of this paper is to see what information could be gained from a minimal number of parameters. PRESUPPOSITIONS AND WORKING HYPOTHESIS

We will start from the following basic facts and arguments: The trophic level of a lake can be expressed in terms of several more or less interrelated measurements, e.g. primary productivity, water transparency, chlorophyll-a content, algal volume, concentrations of nutrients (N and P) and type of community of fish and bottom fauna. These individual means of expressing trophic level only tell us part of the entire "'trophic stort", and they all show different types of relationships with the sediments (Hutchinson. 1957, 1967. 1973; Wetzel, 1975). The average composition of planktonic materials is Ci06Ni6P, which gives a "natural" C/N ratio of 5.6. Phosphorus is generally distributed in a very complex manner in lake sediments, since P may appear in various chemical forms, which may be differently mobile and dependent on the sediment "climate" (e.g. pH and Eh; see Williams et al.. 1976: Bostrrm et al., 1982). Thus, a priori, one may assume that the P-content of lake sediments should not provide the same resolution of the trophic level of the lake as the N-content and,or the C-content.

The elemental composition of humic sut~stances varies roughly accordingly (Gjessing, 1975: Goiterman, 1975): C: 45-60~, O: 30-40~, H: 3-5'>-o, N: 3 - 5 ~ , P: about 0.5~ and Si: about 0.57/o (of the dry weight). This gives a C/N ratio for humus in the range of 10--20. which is significantly higher than for plankton. Thus, the C/N ratio of lake sediments may be used as a criterion to distinguish humosity of lakes (Hansen, 1961). The predominantly minerogenic deposits, like sand, varved (glacial) clay, igneous rock and soils, have a low organic content and low concentrations of N and P, but generally high or very high C N ratios. This is illustrated in Table I. and means that many oligotrophic lakes, which are dominated by minerogenic materials, would have sediments with high C/N ratio and low organic content. Lakes dominated by humus would also have sediments with high CfN ratio, but here the organic content would be high. The sediments contain the following three major (in this context) components: the organic matter, which can be expressed in a simple but crude way by the loss on ignition, the minerogenic matter and the inorganic biogenic material, which is made up of the diatom frustules and biogenic precipitated calcium carbonate. From these premises. Hansen (t961) used the difference in minerogenic matter and inorganic biogenic matter as a measure of the level of oligotrophy to eutrophy; in otigotrophic lakes the minerogenic component is large and the content of inorganic bioganic matter low; in eutrophic lakes the opposite is valid. He also used the C,N ratio as a rough measure of humosity; dystrophic (or polyhumic) lakes should have a C/N ratio larger than 10-15 (by definition dys-trophic means rich in humus); oligohumic lakes should have C tN ratios lower than 10. It is important to account for sample representativity and bottom dynamics, The sediments should preferably be taken out in 1 crn thick layers and not in thicker layers, which yield low resolution (H~kanson, 1981a); thinner layers require special eqmpment (Niemistr, 1974; Fast and Wetzel, 19741. Samples from zones of erosion and transportation may yield

Lake trophic and sediment relationships

305

sequently, the sediments would only provide a longlow differentiating effect as compared to samples term average of the trophic level of a lake. from areas of accumulation (H~.kanson, 1981b). These are the presuppositions, and the ~*orking In this context, the following nomenclature conhypothesis is as follows: cerning bottom dynamics is adopted (H~kanson, The relationship between N-content and IG1977a, 1982a): Accumulation areas prevail where fine materials content of surficial (0-1 cm) lake sediments can be (medium silt and finer) can be deposited con- used as a tool to quantitatively express the trophic tinuously. Transportation zones appear where there is level and the humic level of a lake. Subsequently, we will scrutinize the validity of this a discontinuous deposition of fine particles/ aggregates, i.e. where periods of accumulation (often working hypothesis and define limitations. Two rather long) are interrupted by periods of trans- different sets of data will be used. It is obvious that portation (generally of short duration in connection it would have been beneficial ira full cover set of data from a large variety of lakes were available on several with storms, water turnover or turbidity currents). Areas of erosion prevail where there is no deposition water indicators of trophic and humic level as well as a comparable set of sediment data. That is, however, of fine materials. Areas of erosion are characterized by hard or not the case, and this means that this approach must be regarded with due reservation. Many lakes consolidated deposits; from bare rocks, gravel, sand to glacial clays. The sediments within the areas of throughout the world have been investigated for transportation are, for natural reasons, very variable; trophic indicators; rather few lakes, however, have from sand to gyttja. The deposits within the areas of been studied to meet all the requirements that have accumulation are always comparatively loose, with a to be met in this context; fewer still are available to high water and organic content and sometimes with the author. To explore the large-scale relationships between lake type and sediment type, I have collected a high content of pollutants. It is well established that there exists a strong data from 71 lakes, predominantly from Scandinavia, but also from other parts of the globe. These lakes positive relationship between nitrogen content and organic content (or loss on ignition) in surficial represent most variations in terms of trophic and sediment samples from different sites in a lake (Mack- humic level. This set of data, which is given in Table ereth, 1966; Kemp, 1971). The correlation between 2, will be discussed in the next section. To study the mechanisms between lake type and N-content and loss on ignition (IG) is very important in this approach, and to describe this relationship we sediment type more closely, better data are needed. will define two measures describing two character- Such data are available to the author from 12 Swedistics of the regression line: (1) the slope coefficient ish lakes, and will be discussed in the section entitled will be called BPN (bioproduction number), and (2) "The specific approach". the N-content on the regression line for the IG-value of 10~0 will be called BPI (bioproduction index). It THE GENERAL APPROACH should be stressed that this nomenclature is only The data in Table 2 are heterogeneous. Some lakes utilized for reasons of simplicity to describe these two properties of the regression line and that the BPN- have been investigated in great detail, others less value accounts for what is lake specific, i.e. the value comprehensively. In some of the lakes, the top centiincludes data from many sample sites distributed over meter of sediments was analysed, in others the upper the entire lake area, from shallow waters with low 5cm. The classification of trophic level in oliIG-values to the deep-hole, which generally has the gotrophic, mesotrophic, slightly eutrophic, eutrophic highest IG-value. The BPI-value gives a normalized and very eutrophic is rather rough and made by the measure related to one specific organic content, author on the basis of available information from the references and appropriate literature quoted in these namely IG = 10%. In this context we will not consider the vertical references. Consequently, this information can only variation in sediment cores and the factors be used to explore the major relationships. All these influencing N, C and IG in this dimension (Mack- data have been put into Fig. 1. The lines have been ereth, 1966; Bloesch, 1977; Nikaido, 1978). The focus drawn to maximize the distinctions between the here is entirely on surficial sediments and the areal classes. From this figure two main conclusions can be dimension. It is evident that the information value of drawn: When the organic content is larger than 20-30% surficial sediment is influenced by several factors, like rate of sedimentation, resuspension, bioturbation, (per dry sediments, ds), the relationship between degradation, mobilization, etc., and that the sedi- N-content (total-N) and IG-content cannot be used to ments from a defined layer (0-1 cm) yield time- differentiate lake trophic level. When the organic content is less than 20%, there integrated information. This time-resolution is, genseems to exist a marked relationship if we utilize the erally, primarily governed by the rate of sedimentation, which varies areally and temporally in slope coefficient, as indicated by the lines separating the lakes of different trophic level in the figure. The a lake, implying that the 0-1 cm layer would repre3 oligotrophic lakes with IG-contents lower than 20% sent a different time-span at different sites. Con-

;~)~

La~s H,~KANSON

Table 2. Information on trophic level and sediment concentrations of nitrogen I Nt and los on ignition (IG) Lake Edssj6n Oxudundasjdn S. Bergundasj6n Fioten Frejen Hinnasj6n Trummen Bysj6n Erken Vadsbrosj6n Fysingen Vallentunasj6n Brosj6n S6toftasj6n Ringsj6n Finjasj6n Vatlentunasj6n Tfikern Str~'lken Sk/irhultssj6n Ubby L~.ngsj6n ~)rsj6n M6rkes6 Bavelse $6 Ugles6 Hampen $6 Slauens6 Almind $6 Borres6 Juls6 Grane Langs6 Gribs6 Hj/ilmaren V/inern V/ittern Ekoln G6rviiln Kungszlra Bay VS,ster,.is Bay Blacken Bj6rken Bysj6n V~.sman Ovre Hillen Nedre Hilten Leran Haggen Norra Barken S&lra Barken Noren Triitten Saxen Snyten Stora Aspen Lilla Aspen ~m.~nningen Virsbosj6n Stora Nadden Lilla Nadden Gnien Magsj6n Ostersj6n V,~stersj6n Freden Norrviken J/irlasj6n Stugsj6n Hymenjaure Lucerne Rotsee Mirror Lake

TL

iG ('),) ds)

N (mgg -~ ds)

VE E VE M SE E VE VE SE E E VE E E E E VE E M E M SE SE VE M M SE SE SE E O O VE 0 O SE SE SE E SE M M M SE SE SE M SE SE SE E M SE SE SE E E E E SE SE E E E VE E O O M VE O

t8.4 18.0 48.5 68. I 48~7 69.2 69,4 46.0 11.6 16.6 14.3 27.8 22.2 20.8 22.9 22.2 26.8 15.4 15.0 3718 30.0 42.0 90.5 20.6 81.0 45.3 33.0 24,4 27.5 30.2 11.7 55.0 12.3 9,7 11.9 7.9 9.9 7.0 8.6 9.9 11.6 13.1 14.7 12.6 10.0 8.1 14.6 11.5 8.9 12.5 17.5 I 1.0 9.4 9.6 11,5 9.5 8.5 9.8 9.4 14,0 9.8 12.0 22.6 10.9 23.1 31.0 43.0 31.0 I 1.0 18.0 28.3

1.25 1.18 3.48 1.46 1,76 1.20 1.24 3.06 0.62 0.74 0.88 1.35 1.17 I. t 5 1.31 1.26 1.62 0.89 0.65 1.5 1.3 1.5 3.0 0.9 2.7 2.6 1.3 I.I 1.4 1.5 0.35 2.45 0.9 0.27 0.36 0.38 0.52 0.35 0.42 0.50 0.49 0.51 0.49 0.69 0.52 0.35 0.52 0.63 0.43 0,62 0.97 O.51 0.48 0.44 0.50 0.66 0.52 0~75 0.63 0.62 0.52 0.79 1.44 0.54 1.47 1.52 1.40 1.20 0.55 1.l l.t0

Reference Hillerdal ( 1975 Hillerdal (19751 Bengtsson et al. (19771 Bengtsson et al. (19771 Bengtsson e t a [ . (19771 Bengtsson et al. (19771 Bengtsson et al. (19771 Bengtsson et al. (19771 Bostr6m and Pettersson (19821 Bostr6m and Pettersson 1"t9821 Bostr6m and Pettersson (1982) Bostr6m and Pettersson (19821 Bostr6m and Pettersson ([982) Bostr6m (19821 Bostr6m (19821 Bostr6m (19821 Bostr6m (19821 Bostr6m (19821 Hansen (1961) Hansen (1961) Hansen (19611 Hansen (19611 Hansen (19611 Hansen (19611 Hansen (1961) Hansen (19611 Hansen (19611 Hansen (1961) Hansen (19611 Hansen (1961) Hansen (1964) Hansen (1956) H~ikanson (1981a) H~,kanson (1981a) H~kanson (1981a) H~.kanson (1981a) H~.kanson (t981a) H~kanson (1981a) H~ikanson (1981a) H',Ikanson (1981a) H;lkanson and Uhrberg (198t) H~,kanson and Uhrberg (!981) H~'tkanson and Uhrberg (!98t) HAkanson and Uhrberg (1981) Hstkanson and Uhrberg (1981) H:ikanson and Uhrberg (1981) H:lkanson and Uhrberg (1981) Hfikanson and Uhrberg (19811 H,'ikanson and Uhrberg O9811 H~kanson and Uhrberg (198t) H~tkanson and Uhrberg (19811 H~.kanson and Uhrberg (19811 H'.tkanson and Uhrberg (19811 H~tkanson and Uhrberg (1981) H~.kanson and Uhrberg (19811 H:Ikanson and Uhrberg (1981) H~ikanson and Uhrberg (19811 H~tkanson and Uhrberg (19811 H~ikanson and Uhrberg (1981) H~ikanson and Uhrberg (19811 HAkanson and Uhrberg (1981) Hlikanson and Uhrberg (1981) H~Ikanson and Uhrberg (1981) Hllkanson and Uhrberg (19811 Dietrichson (19761 Bengtsson et al. (19721 Lithner (19741 Lithner (19741 Bloesch (t977) Bloeseh (1977) Mocller and Likens (19781

VE = very eutrophic. E = eutrophic. SE = slightly eutrophic. M = mesotrophic. 0 ~ oligotrophic.

Lake trophic and sediment relationships 3z

-

,b

307

3 48

0 +

24--

/

E

1.6 --

//e

:; il " /'/ ,'*I

t~

:~

g

/

/I/

•

"

°

20

T

+ OI igotrophic o Mesofrophic

"/./,'" .'1/

Lt
O

+

• Slightly eutrophic • Eutrophic • Very eutrophic

I 40

I 60

I 80

I IO0

Loss on ignition (IG,% ds)

Fig. I. Nitrogen content vs loss on ignition in sediment samples from 71 lakes of different trophic level. are all correctly identified beneath the lowest line. Five of the 7 mesotrophic lakes are correctly identified and 2 are classified as slightly eutrophic, which is as close as one could get. All 16 lakes classified as slightly eutrophic are correctly identified between the two given lines marked M and SE. Six of the 12 eutrophic lakes are correctly identified; 3 are plotted as slightly eutrophic, 3 as very eutrophic. Two of the 3 very eutrophic lakes are correctly classified and the remaining on is eutrophic. Considering the reliability of some of available data, this is a very promising result, which supports the working hypothesis and indicates that the BPN-value could be an adequate sediment measure of lake trophic level-but only when the organc content is less than 20~o. In dystrophic lakes, which generally have much higher IG-values, the BPN-approach would not be valid. THE SPECIFIC APPROACH

The aim of this section is to give a more thorough discussion on the relationship between lake trophic type, as determined from a comparatively large set of water indicators, and sediment type, as determined from surficial sediment data on N-content (total-N) and loss on ignition (IG). The goal is to obtain a long-term average (in the range 5-10 years) determination of lake trophic level from simple sediment data.

Water data Presently, l have access to data indicating trophic level on: transparency (Tr in m), chlorophyll-a (Ch in

mgm3), totaI-P (P, m g l - t ) , total-N (N, mgl-~), organic-N (ON, mg I -i), inorganic-N (OON, mg I -~) and also values on the mean depth of the lakes/basins (D, m). All these raw data emanate from unpublished material collected by the National Swedish Environment Protection Board, Water Laboratory, Uppsala, Sweden. From these basic data the following ratios have been established: total-P to mean depth (P/D), total-N/D, organic-N/D and inorganicN/D. Mean values based on monthly samples from the vegetation period (May-Oct.) for the period 1970-1977 have been determined for the following Swedish lakes/basins: HjS.lmaren, Great-Hj/ilmaren, V/inern, V~ittern, Ekoln (a basin in northern Lake Miilaren), G6rv/iln (in central Lake M/ilaren), V'/ister~.s bay (western Lake M/ilaren), Blacken (western Lake M/ilaren), Northern River Kolb/ick (Lakes Bysj6n, Bj6rken and Haggen), Middle River Kolb~ck (Lakes Leran, N. Barken and S. Barken) and finally Southern River KolbS.ck (Lakes Magsj6n, (3stersj6n, V/istersj6n and Freden). Water indicators are given in Table 3. It should be stressed that none of these lakes/basins belong to the dystrophic type. It is clear from any textbook in [imnology that single indicators like transparency, chlorophyll-a, or the given ratios, can only provide a more or less rough estimate of the trophic level, especially if the available data are scarce. A weighted index should give a more balanced and accurate description. Therefore, to obtain a more representative picture of the trophic level in the actual 12 Swedish lakes, and to circumvent the problem of representativity, I will define a TL-value (trophic level) based on four of the most reliable and traditional indicators (Tr, Ch, P/D

!l)S

LARS

HAKasSOX

and N/-D), i.e.: T1 = f ( T r , Ch, P/D, N,D 1.

~e ~.,==

=_~

5~ O× , ~

'~0:

~,

~

~,~ - , ~

=

: . . . . .

•r- O

,o,--

Z e",7,

z

~.~

.Y. -~

~z

Z

-"[

o~=

,o

~ g :

{ l)

It should be emphasized that TL is not introduced as a new, general, water indicator of trophic level. merely as the best possible relative measure against which the available sediment data from the 12 Swedish lakes can be calibrated. It is the sediment indicators and not the water indicators that are in tbcus in this work. The reasons for utilizing the quotients P/D and N/D and not N, P, ON or OON, is that these ratios often provide a better description of the trophic level, since the mean depth is an important governing factor for lake productivity (Rawson, 1955~ Vollenweider, 1968; Wiederholm, 1979; Saether. 1979). The TL-value has been determined accordingly: The value indicating highest trophic level for each variable is transformed to 100 to obtain comparability, i.e. transparency 13 m for V~ster'as Bay, chlorophyll-a 23.0 mg m -3 for S. River Kolb/i.ck. P/J~ 72.104 for S. River Kolb~ck and N.D 138.10 for S. River Kolb/ick. The obtained values are given within brackets in Table 3. Then these percent values were summed for each lake, see the column marked TL*. Finally, the TL*-values were redimensioned by putting the highest value ( T L * = 387 for S. River Kolb~ick) to 100. The result is a ranking of the given lakes in terms of oligrotrophy and eutrophy (and not in terms of dystrophy). The obtained Tl-values vary from 7 for Lake V/ittern, which is most oligotrophic according to this approach, to 100 for S. River Kolbfick, which is most eutrophic. This ranking seems to correspond with the general knowledge of these lakes (see SNV, 1975; H:~kanson, 1981a). Subsequently, the aim is to see how the TL-value, and the other water indicators of trophic level, are correlated to each other and, more importantly in this context. to the sediment indicators.

Sediment data o,&

.~

.

o-~

The actual set of sediment data from the 12 Swedish lakes/basins is given in Table 4. It comprises the following parameters: Mean values from areas of accumulation tbr organic content (loss on ignition IG in ~, ds = dry sediments), carbon (estimated as IG/2, °/ O/ d s ) . nitrogen (N, ~ ds), phosphorus (P, ~,~ ds), and the quotients C/N and C/P. Characteristic values (H~,kanson, 1981b) for the same variables, marked IG~, Ca, etc. Two indices: BPN and BPI, which are defined by the relationship between the IG-content and the N-content of surficial sediments (0-1 cm). RESULTS From the introduced data, we will now study how the given indicators of trophic level co-variate. The result of the correlation analysis is given in Table 5.

(B)

(A)

TL

Pill NID

C'h

Tr

()ON/l) ON/D

I(]~ CA Nj, Pt, CdN~ BPI !|PN D

C/P

IG C N P C/N

~ s,

1.00

10.46 2.05

IG

20(35) 101151 84(1011 41 (91) 12 27(32) 14(161 25 (28) 23 (24) 10 14 1615)

n 12.3 12.5 9.7 11.9 7.9 9.9 7.0 8.6 9.9 13.0 9.8 13.0

6.2 6.3 4.9 6.0 4.0 4.9 3.5 4.3 4.9 6.5 4.9 6.5

0.90 0.85 0.27 0.36 0.38 0.52 0.35 0.42 0.50 0.50 0.48 0.71

0.21 0.24 0.15 0.13 0.09 0.19 0.14 0.20 0.22 0.16 0.19 0.16

6.9 7.4 18.1 16.7 10.4 9.4 10.0 10.2 9.8 13.0 10.2 9.1

29.5 26.3 32.7 46.2 44.4 25.8 25.0 21.5 22.3 40.6 25.8 40.6

13.9 14.0 11.5 15.5 8.7 10.8 8.5 9.2 10.5 16.0 11.0 16.1

7.0 7.0 5.8 7.8 4.4 5.4 4.2 4.6 5.3 8.0 5.5 8.1

0.86 0.86 0.30 0.45 0.39 0.48 0.43 0.43 0.52 0.63 0.53 0.84

0.25 0.29 0.17 0.15 0.10 0.24 0.18 0.23 0.22 0.17 0.21 0.20

Characteristic values IGj, Ci, N~ P~, _(°./od_s! (~__ds) (% d s ) . ( % ds) 8.1 8-1 19.2 17-2 11.2 11.3 9.9 10.7 10. I 12.7 10.4 9.6

CUN,l 0.73 0.93 0.86 0.78 0.81 0.75 0.92 0.76 0.72 0.96 0.85 0.97

rm N 0.71 0.85 0.20 0.67 0.41 0.53 0.56 0.74 0.40 0.29 0.77 -0.03

rio,, e

re N 0.69 0.73 0.20 0.72 0.05 0.41 0.58 0.69 0.26 0.06 0.69 -0.10

Correlations 6.8 6.8 2.9 2.9 4.9 4.7 4.9 4.9 5. I 3.8 5.0 5.0

1.00 1.00

5.24 1.03

1.00

1.00

0.96 0.96 0.47 0.11 0.15 0.53

1 0 . 9 3 31.72 12.11 3.40 8.90 2.81

0.65 0.34 - 0 . 0 4 0.36 0.65 0.34 - 0 . 0 4 0.36 1.00 I}.63 0.73 - 0 . 1 5 1.00 - 0 . 5 3 - 0 . 7 3 1.00 0.46

0.52 0.17 0.20 0.04 5.60 1.94

0.96 0.76 0.96 0.76 0.47 0.96 0A I 0.53 0.15 - 0 . 6 4 0.53 0 1.00 11.63 1.00 0.63 1.00

6.07 1.42 0.33 0.33 0.70 0.95 -0.59 -0.71 0.13 0.13 0.60 1.00

2.01 0.51 0.05 -0.05 -0.70 -0.50 0.99 0.44 0.12 0.12 -0.65 -11.55 1.00

11.52 3.42 0.12 0.12 0.82 0.61 -0.94 -0.46 -0.09 -0.09 0.70 0.67 -- 0.92 1.00

4.81 1.22 0.21 0.21 0.77 0.33 -0.84 -0.18 0.08 0.08 0.74 11.46 - 0.83 0.83 1.00

0.49 0.15 -0.12 -0.12 -0.62 -0.50 0.78 0.43 0.02 0.02 -0.63 0.49 0.83 -0.76 -0.80 1.00

15.47 10.37

-0.51 -0.21 0.03 -0.07 -0.42 -0.22 -0.56 -0.56 0.08 - 11.10 -- 0.43 0.34 1!.37 -0.43 1.00

33.75 22.80

1.00

0.32 0.32 0.83 0.42 -0.78 -0.18 0.18 0.18 0.81 11.51 - 0.78 11.83 0.92 0.81 0.45

45.50 31.34

0.32 0.32 0.29 0.30 0.69 (}.53 0.47 0.47 -0.23 11.31 0.69 0.62 11.69 11.84 - 0.63 11.59 1.00

3.10 2.36

1.00

(}.69

0.05 0.05 I}.57 I).21 0.75 -0.15 -0.06 -I}.06 (}.55 0..t2 I).74 0.68 11.911 11.711 11.65 0.86

10119 6.51

-

I 1111

0.117 0.07 11.66 0.36 I}.811 11.28 11.06 0.06 OAi3 0.44 - 0.81 11.81 0.91} 0.80 0.65 (}.95 0.69 (I.93

37.44 25.80

1.1111

(l.tll} ().t)~-i

0.03 0.03 I}.55 0.26 11.7.t (}.2.t 0.15 -0.15 11.51 11.311 I).74 0.74 11.81 11.76 11.79 11.91} 11.71

79.25 46.38

N/D

0.70 0.61 0.29 0.24 0.52 0.45 0.49 0.54 0.38 0.41 0.49 0.71

Indices BPI BPN

Table 5. (A) Mean values (7~') and standard deviations Is,) for various indicators of trophic level from 12 Swedish lakes/lake systems. (B) Correlation matrix C N P C/N C/P IG~t C~, N~, Pj, Cj,/N~, BP[ BPN D OON/D ON//) Tr Ch P/D

Hj/ilmaren Great Hjiilmaren V/inern V:'ittern Ekoln Gfrviiln Kungsilra Bay Viisterfis Bay Blacken N. River Kolblick M. River Kolb/iek S. River Kolb-~ck

Lake/lake system ....

Mean values from accumulation areas IG C N P (% ds) (% ds) (% ds) (% ds) 12/N C/P

Table 4. Sediment data indicating trophic level from 12 Swedish takes/lake systems. Mean values from accunlulation areas and characteristic wilues given in percent of dry sediments (~.~ ds) lbr the organic content (IG), the carbon content (C), the nitrogen content IN) and the phosphorus content (P). The number of samples from accumuhition areas and the total number of sediment samples from the various lakes/lake systems are given in the column marked n. The correlation coellicients have been determined fronl all data from the lakes. For further details about this material see |llikanson (1981a)

1.110

11.98 0.98

0.9(i

0.16 0.511 II..t.t 0.77 0.72 0.81fl 0.78 0.76 O.g9 -11.75

016

-005 - 11.115 0.51 11.27 - (I.7& 11.27

54.00 30.50

TL

o

lq

laa

g, o ,i:Ii

r-"

, It)

EARS

Z !¢"

7

z" c;

z" 21

Z

Z Z

7-

Z

(,

:e z ";5 m~

L

5~J ~a

e, ~2

,.6

..z ~ > = ~ , = : ~

:~z z ~

H~KANSON

Mean values (2) and standard deviations ts~t for the variables from the 12 lakes/basins are shown under (A) and the correlation matrix under (B). From this table it is clear that the BPN-value provides the best correlation with the TL-value and with the other water indicators of trophic level. We may note: That the various water indicators, as expected, show a high degree of internal co-variation and, naturally, a good correlation with the TL-value. That the solitary sediment indicators (IG, C, N, P, IGk, Ck, Nk and Pk) do not provide as good information as the indicators based on both the N-content and the IG-content (BPN, BPI, C/N and C~/N~). That the BPl-value and the quotients C/N and Ck/Nk constitute a group of sediment indicators that also give satisfactory information about potential trophic level. That all sediment indicators based on P, as expected, provide a low correlation with TL. The accuracy of the BPN-value as compared with some of the other sediment indicators may also be demonstrated by a simple complementary rank test, using the Spearman formula (see, e.g. Spiegel, 1972). Applying this formula stresses what has been said about the TL-value as a weighted, relative measure to be used only for calibration purposes in this context. The test is illustrated in Table 6. The lakes have been ranked first according to the TL-values, whereby S. River Kolb/ick with the highest TL-value (100) is given the rank value (R) I and Lake V/ittern, the most low-productive lake, is given the R-value 12. Then all lakes/basins have been ranked in the same manner according to BPl-value (RBPI), BPN-value, C/N-ratio and Ck/N,-ratio. The absolute differences (D) between the rank values, the sum of these D-values and the coefficients of rank correlation have been determined. It is clear from Table 6 that the BPN-values gives the least error (8) and the best fit (rr~,k = 0.96), and that the C/N-ratio provides the worst fit compared to the TL-va[ues. These results may be concluded in the following manner: The slope of the regression line between the Ncontent and the IG-content of surficial sediment samples (0-1 cm) providing an even area cover of the lake bottom seems to be the best sediment measure of lake trophic level in lakes dominated by sediments with IG-contents lower than 20%. The other measures related to this regression line (BPI, C/N or IG/N, C~/Nk or I G J N , ) also gives substantial information about take trophic level. Subsequently, we will discuss some important causal relationships behind these results and try to answer the question: why will BPN yield best resolution? CAUSAL

RELATIONSHIPS

In this section we will focus on three issues that have to he accounted for to answer the given ques-

Lake trophic and sediment relationships (N, % ds)

167 ==

A

TROPHIC LEVEL / V E ; N.O.70.IG-O.16

/

Table 7. Data on lake constant (Kt.L characteristic loss on ignition (IG~), "'critical" water depth {Dr -0 and "critical" loss on ignition 4IGr ~ in some Sv,edish lakes Ifrom Halkanson. 19~31b) Lake

;

t~

.~e~.. ,o~..E: N.O.Sr.~-Cte4

[

]

~

O

4

N,O.24.1G*0.42

;

12

2 0 (IG, % ds)

Loss on ignition 4-

~1

B

l

-

31 I

Hj/ilmaren V/inern N. Lake VS.ttern W. Lake M/ilaren Ekoln

Kc

[G~

Dr a

[Gr :~

2.0 7.0 4.5 20.0 3.0

13.5 11.5 12.0 11.0 11.5

11.7 28.8 46.7 21.7 16.2

9.3 7.8 9.9 6.1 ".4

tuting the lake Mfitaren system) generally is less than IG = 10°~. This is calculated from the formula (see H~.kanson, 1981b and Table 7).:

l ~ o

IGT.~,

.H I ~

=

IGk --

100 -- DT. a

KL D r . , + KIc" KL

(2)

where ) :z -

2-

r ~ t e r ' a s bay ~ '~ Hjalmaren 3El.T

E VE

F

IGT.A i

0 Ero,%-~-Transp. -,. Acc. 4 1'2

2'0 (IG. % ds)

Loss on ignition

Fig. 2. The relationship between nitrogen content and loss on ignition in surficial sediment samples from four lakes of different trophic level. tion. The first issue concerns interpretations of data from the given regression line, the second question deals with the characteristic values and the last problem has to do with limitations of this approach and the transition zone. Regressions To clarify the concepts, Fig. 2 gives the regression lines for four selected lakes of varying trophic level: oligotrophic Lake V/ittern, mesotrophic Lake M/ilaren, eutrophic VS.ster~.s Bay and very eutrophic Lake Hj/ilmaren. These lines show very nicely how the slope coefficient (BPN) increases with increasing trophic level. But these lines do not cross the origin, implying that the lines will change shape rather drastically if we put the quotient IG/N (or C/N) on the y-axis instead of the N-content, and keep the IG-content on the x-axis. This is illustrated in Fig. 2(B). A regression line crossing the origin in Fig. 2(A) will be transformed to a straight line crossing the ).-axis at I/BPN in Fig. 2(B). The character of the regression line depends on many things: sampling (here we may assume similar sampling in the 4 given lakes), type and character of the settling particles, i.e. content of plankton, humus and minerogenic matter with different N-, C- and IG-content, and prevailing bottom dynamics, i.e. the areal distribution of zones of erosion, transportation and accumulation, which is related to potential resuspension activity (H~kanson, 1982a,b). Here it may be sufficient to indicate that the "critical" limit separating areas of accumulation from zones of transportation in these 3 lakes (ViisterS.s Bay belongs to the archipelago system consti-

IGT_, = the "'critical"organic content (loss on ignition), in "j.ods: IGe = the characteristic IG-content. in 3'0 ds; DT.A = the "'critical" water depth, in °o of the maximum depth; K L = the lake constant: Kta = the parameter constant, which is 15.0 for IG. The main conclusion that can be drawn from Fig. 2 is that the slope coefficient (BPN) accounts for the areal spread and the prevalent bottom dynamics to yield a lake specific sediment measure of trophic level, whereas any point on the regression line only yields site specific information. In this context, it is important to emphasize that analogous results would be obtained if, e.g., the carbon content is compared with the loss on ignition (see also Mackereth, 1966). Characteristic values Equation (2), and the theory behind this formula, accounts for the fact that various physical sediment parameters, e.g. water content, bulk density, grain size and organic content, and chemical parameters, e.g. nitrogen, are distributed in a very typical way in lakes (H:~kanson, 1981b): The distribution depends on the water depth. The spread around the mean is generally large in shallow regions where erosion, transportation as well as accumulation of fine materials may appear. The spread decreases with increasing water depth. The spread is relatively small at all water depths within the area of accumulation. This allows the determination of a characteristic single value for the lake at the maximum depth (Dm,x = Dp = 100% ). The water content of surficial sediments (W0_0 is regarded as a key sediment parameter; most physical and non-contaminating chemical sediment variables may be related to the water content. It should be stressed that characteristic values can only be determined according to this method for lakes which have areas of accumulation. Contaminating substances are often distributed in lake

312

L~,s H~.IO.NSON

by IG-values smaller than 20°0 can. seemingly, be classified by the BPN-value, and lakes dominated by IG-values larger than 303/0 cannot be classified for trophic level by the BPN-approach. These latter lakes (%} n (?o} (% ds) {% ws) are generally dystrophic. But what does dominated 20--30 7 24.1 18.8 9.0 81.4 14.0 mean? 30-50 7 44.4 20.4 9.I 86.2 15,7 50-70 5 61.1 20.6 6.2 91.0 4.9 Lake Norrviken clearly lies in the transition 70--90 5 81.6 29.9 2.8 94.7 0.5 zone. This is emphasized by the data ~ven in Fig. 3. 90.-100 4 94.6 30.2 1.7 95.4 0.5 In this figure we can see that the correlation between the N-content and the IG-content is higher for samsediments in the form of distinct lobes with deples with IG-contents smaller than 20°o ~r =0.97) creasing concentrations with distance from the source than for sites with IG-contents higher than 20% of pollution, and for such substances the method (r = 0.85). Consequently, the 959,0 confidence limits. cannot be applied Neither has the method been as determined according to standard procedure tested on small (a < 1 km 2) lakes. In, for example, (2"Sh=S~'x/1 r', see. e.g. Gregory, 1963) lie close dystrophic lakes one would generally obtain very to the regression line in the first case (2.s_ = 2.30) high water contents (in the order 95-99%), very high and further apart in the latter case (2.s, = 3.57). organic contents and, hence, also very high "critical" From the hysographic curve for Lake Norrviken, see water and organic contents; at least much higher than Ahlgren (1967), we can determine that 51.2% of the 10%. High organic contents can also be found in, e.g. lake area lie above the "critical" limit (Dv.A = 4.7 m) eutrophic lakes, as illustrated in Table 8 with data and belong to the zones of erosion and transfrom Lake Norrviken, Sweden (see Ahlgren, 1967, portation. And from the data given in Table 2 on 1977; Dietrichson, 1976; UI6n, 1977). The "critical" Lake Norrviken, and from the position of this lake loss on ignition (IGD is 30.3%, which is significantly in Fig. 1, we may conclude that Lake Norrviken lies higher than in the large Swedish lakes illustrated in just on the limit where the BPN-approach can be Fig. 2. The data from Lake Norrviken do not ema- used. Thus. as a rule of thumb, it may be said that nate from the 0-1 cm layer but from the 0-5 cm layer. 5070 of the lake area should have 1G-contents lower This would not, however, affect the applicability of than 20~ for the BPN-approach to be applicable as these data to illustrate the principles behind the an indicator of lake trophic level. For lakes domilinkage between concentrations, spread patterns, bot- nated by IG-eontents in the range 20-30%, the tom dynamics and information value of sediment method can be used but with reduced validity. The samples. Another motive for using these data from method cannot be used when more than 50% of the Lake Norrviken in the present context is that this lake surface has higher IG-contents than 30°;]. For particular lake falls into the transition zone, as such lakes the relationship between N-content and indicated in Fig. 1, where the BPN-value may not be IG-content can be used (see Hansen, 196l) to express used to indicate lake trophic level; the IG-content is humosity. Oligohumic lakes will have IG;N ratios generally larger than 2070 below 3.6 m water depth lower than 20, mesohumic lakes, which constitute the (Dp = 30%) in Lake Norrviken (see Table 8). transition zone between oligo- and polyhumic lakes, would have tG/N ratios between 20 and 25. And true The transition zone polyhumic lakes would have IG fN ratios higher than The transition zone has been indicated with two 25. lines at IG = 20 and 30% in Fig. I. Lakes dominated It should be stressed that: Table 8. Base data from Lake Norrvikenfor determinationof lake constant {KL),"'critical"water depth (DT.A)and characteristic loss on ignition(IGD

CN, % ds) 2.4/I"

...'"i';/" ;.--'"""

LAKE NGflRVIKEN N • 0.068,1G- 0.11 n.28 B P N , 6 . 8 ..>V~ry eutrophic r • 0.97,

1.6

,

,~ .'" I t,o.-" J • /"

z

~#~..~-" /

0.8' •\ . ,,"

/"

...'"

1/-

.,/"/ tI

4

-"

../"

N,0.I3.1G-0,23 r . 0 . ~ , n. 20 2 SlG- 3.57

///'/

/"

Sediment layer O-5cm ]

,'"

4,.--'. ~ h . o . o ~ . ~ - o . ~ .">/..'" ,.o.gz ..8

r"

%..'.

IG>20 -> 2 SlG-3.57 IG,c20 -> 2 SlG.2.30

2 siG,2.ao 12

20

~e

~ (JG, "r. es)

LOSS o n ignition

Fig. 3. The relationship between nitrogen content and loss on ignition in surficial sediments in Lake Norrviken, Sweden. (Base data from Dietrichson, 1976.)

Lake trophic and sediment relationships

313

32

IG / ~" / /20

TROPHIC TRAN$ LEVEL ZONE I

-

J /HUMIC 2 5 / L _ . . E V__ EL

/

z

/

§

,

'

.,m.o,s

8PN---" / /

='"

I

Z 08

.~

I 0

6.511/'/I

BPN

45 / / 3

Ninmgg-lds G n%ds

o-,

~

]

" I--'G--

/ ~o

40 60 80 I00 LOss on ignition (IG,%ds) Fig. 4. Diagram illustrating the relationship between nitrogen concentration and loss on ignition of surficial sediments (0-1 cm) relative to lake trophic level and lake humic level. 30

The dystrophic lake type is not fundamentally different from the oligotrophic-eutrophic series of lake (Hansen, 1956). In nature there are only smooth and continuous transitions between various lake types. Lakes can be, for example, eutrophic, i.e. have a high bioproduction, and polyhumic, i.e. have a high content of humic substances. This work focuses on the trophic and not the humic side of the diagram in Fig. 1, which implies that the given limits are more reliable on the trophic side. These results, as they are summarized in Fig. 4, should be taken for what they are: indicative, preliminary, simplifying. It is probable that Fig. 4 cannot be applied in lakes dominated by periphyton. The sediments reflect and affect the conditions of the lake water, implying that the areal perspective and the bottom dynamics must be taken into account when discussing the linkage between lake trophic level and lake sediments. More and better data may alter some of the given limits: IG = 20 and 30%; BPN = 0.33 as the limit between oligotrophy and mesotrophy, BPN = 0.45 as the limit between mesotrophy and eutrophy, BPN = 0.65 as the limit between eutrophy and very eutrophic lakes; I G / N = 20 as the border between oligohumic and mesohumic lakes and I G / N = 25 as the limit for polyhumic lakes. If only a limited number of sediment data are available from a lake, e.g. from the deepest part, then a regression analysis cannot be made. To conclude, it is the author's opinion that Fig. 4 and the causal relationships behind this diagram provide new insights into the very interesting relationships between lake trophic type and lake sediments.

Acknowledgement--The author wishes to acknowledge the help from an anonymous referee when revisiting the manuscript. REFERENCES

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31"

LaRs HAg~,.~sor,-

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