The trophic status ofthe Laurentian Great Lakes is greatly influenced by phosphorus ... To facilitate predictions for the future of the Great Lakes or any lake ...
J. Great Lakes Res. 17(1):132-140 InternaL Assoc. Great Lakes Res., 1991
PHOSPHORUS FROM INTERNAL SOURCES IN THE LAURENTIAN GREAT LAKES, AND THE CONCEPT OF THRESHOLD EXTERNAL LOAD
Gertrud K. Nornberg
Freshwater Research R.R. 1 Raysville, Ontario POR lAO ABSTRA CT. The trophic status ofthe Laurentian Great Lakes is greatly influenced by phosphorus (P) derived from anoxic sediment surfaces. Data from the Great Lakes and data from smaller lakes of Eastern North America can be used to demonstrate how such an internal P load influences trophic state. To facilitate predictions for the future of the Great Lakes or any lake subjected to P release from anoxic sediment surfaces, the concept of «threshold external load" is introduced. The external P load at which the flux downward from external sources matches the flux upward from anoxic sediments can be considered the «threshold external {oad". The product of the «threshold external load", the gross P retention (predicted from the annual water load) and the ratio of lake surface area to hypolimnetic area (a sediment focusing factor) yields the anoxic P release. The concept of «threshold external load" helps explain the slow response of certain lakes to phosphorus input abatement. INDEX WORDS: Phosphorus, eutrophication, eutrophic lakes, mathematical models.
INTRODUCTION
of an internal load term (Niirnberg 1984) or implicitly by the usage of observed P retention (Dillon and Rigler 1975). It is not possible however to predict the response to these lakes to restoration measures because of the unpredictable behavior of internal load, even though external load, morphometry, and hydrology can often be approximated. Because of this problem, viewing the sediments as a regulator of the content of P in the open water instead of the watershed only is proposed. In this context, internal load can be seen as selfpurification rather than self-fertilization, since it helps decrease the P content in the sediment to a level such that release eventually ceases. To determine if a lake is actually in the state of selfpurification or if the external input still overwhelms the release, a threshold of external load can be defined at which the flux from external sources downward matches the flux upward from the anoxic sediments. If the present external load is lower than the threshold load, the lake is in the state of self-purification and becomes less trophic. Because of the size and importance of the five Laurentian Great Lakes, this study concentrates on them. The study results are applicable, however, to any lake with high sediment-derived P.
Any lake close to human settlement is a candidate for eutrophication. Deterioration of its trophic state will happen although the time of this process may vary with different morphometry and geochemistry of the lake and the kind of exposure to human activity and usage. Even for large and deep lakes as well as the ocean, incidences of high turbidity, increasing presence of obnoxious algae, and elevated P (phosphorus) concentrations have been reported. In many cases management activities have been centered on the reduction of P inputs, since P is or should be the limiting nutrient in most lakes (Vollenweider 1968). However, more and more incidences have been reported lately of lakes that have not or only slowly responded to P abatement (summary in Cullen and Forsberg 1988). In many of these cases, where predictions based on mass balances of incoming P alone (e.g., Vollenweider 1976) underestimated the actual P concentration, P from sources within the lake were responsible for the deviation (Niirnberg 1984). It has become obvious, therefore, that steady state mass balance models apply to the P cycle in lakes with internally derived P only if internal loads are taken into account, either explicitly by the addition 132
PHOSPHORUS FROM INTERNAL SOURCES
133
TABLE 1. Data relevant to the trophic status of the Great Lakes. Average total phosphorus concentration of the water (Water- TP) and the surface sediment (0-10 cm, Sed. - TP), external TP load (L ext), population density (inh, inhabitant), trophy, year of first signs of changes toward eutrophication (Year). Most data are from the late seventies and early eighties. Water-TP' Sed.-Tpb Lext' DensityC Trophyd mg m- 3 mg g dw- 1 mg m-2 yr- 1 inh km- 2 Bay or Lake
Ontario Bay of Quinte Niagara Basin
21 50 na
0.85 1.49 4.3
725 2,350 na
110.9 na na
o-m e e
1820 1669 na
Erie Western Basin Central Basin Eastern Basin
39.3 19.4 17.2
na 0.88 na
3,660 730 820
172.5 na na na
m-e e m m
1880 na na na
Michigan Green Bay
8.0 na
0.75 na
100 1,260
127.3 na
o-m e
1970 na
Huron Saginaw Bay
5.5 40.0
na na
82 400
18.6 51.1
0
na na
m-e
5.5 Superior o na 50 4.6 na na, data not available. 'Janus and Vollenweider 1981; Saginaw Bay, Bierman and Doland 1981; Bay of Quinte, Minns 1986; Minns et ai. 1986. bMudroch 1983. CBangay 1981. dO, oligo-; m, meso-; e, eutrophic; Auer et ai. 1986. eSchelske et ai. 1983; Schelske et ai. 1985.
The Trophic Status of the Great Lakes Based on average total P concentrations (Vollenweider 1968) and experiments involving the response of algal growth rates to P additions (Auer et al. 1986), the upper lakes (Superior and Huron) can be classified as oligotrophic, Lake Michigan can be classified as oligo- to mesotrophic, and the lower lakes as meso- to eutrophic (Table 1, also Dobson 1981). Signs of eutrophication based on biogenic silica distribution in the sediments have been found for as early as 1669 (Schelske et al. 1985, Table 1). Only recent or no indication of eutrophication can be found in the main basins of Lake Huron and Lake Superior. That the origin of the eutrophication of the lower lakes is anthropogenic can be seen by the densities of the shoreline population as well as by the size of external P load (Table 1). Extended periods of anoxia during summer stratification have been noted in Lake Erie, whose shallow but stratified central basin (57010 of surface area of total Lake Erie) can become anoxic for several months in part of the hypolimnion (Fig. 1,
Burns and Ross 1972). Of the other deeper lakes, hypolimnetic water or sediment surfaces of bays with high TP loading have been observed to become anoxic. Examples include the Bay of Quinte (1.4% of Lake Ontario area), Hamilton Harbour (ca. 1% of Lake Ontario area), and part of the Niagara basin (ca. 0.5% of Lake Ontario), Green Bay (0.6% of Lake Michigan), and possibly Saginaw Bay (5.8% of Lake Huron). In all of these areas, P return from the sediment has been observed (Burns and Ross 1972, Minns 1986, Conley et al. 1988).
Quantities of Internal P Load and Release Rates Indirect estimates of internal P loads (Lint' mg m- 2 of the lake surface area yr- 1) were derived from P mass balances for central Lake Erie (129 mg m- 2 yr- 1; Burns and Ross 1972) and Bay of Quinte (2,070 mg m-2 yr 1; Minns et ai. 1986). While the amount of Lake Erie is small and represents only one sixth of the external load, Lint of Bay of Quinte is almost as high as the external load to the bay (Table 1).
134
G.K.NURNBERG
Boy of Quinte
Superior
FIG. 1. Map of the Great Lakes indicating the approximate areas of seasonal anoxia of the sediment surfaces (shaded) in Green Bay, Lake Michigan; Saginaw Bay, Lake Huron; central basin of Lake Erie; Hamilton Harbour and Bay of Quinte, Lake Ontario. Bay of Quinte is enlarged with sampling stations indicated as: H, Haybay; P, Picton; G, Glenora; C, Conway.
In general, Lint depends on the spatial and temporal extent of anoxia, which can be summarized (see Niirnberg 1987) as the anoxic factor (AF) (equal to duration of anoxia * anoxic sediment area / lake surface area; d yr- 1) and the areal rate of P release (RR) (mg m- 2 of anoxic sediment surface d- 1 of anoxia). Thus, Lint
=
AF
* RR.
(1)
P release rates are highly variable (0 to 50 mg m- 2 d- I ), are related to the trophic state of lakes (Niirnberg 1988), and are significantly correlated with the average lake TP concentration (Equation in Table 2; Niirnberg et al. 1986). In addition, highly
significant correlations have been found with sediment P concentrations (Equation in Table 2; Niirnberg 1988). Only a few RR have been measured in the Great Lakes, but more can be predicted from the relationships with average TP concentrations in the water and the sediment (Table 2). The prediction of RR is particularly important in areas where its experimental determination as the accumulation of hypolimnetic P is impossible. This is the case in bays whose hypolimnia are connected to the open water (e.g., lower Bay of Quinte and Green Bay), or in areas with weak stratification where the water becomes anoxic only intermittently while the sediment surface remains anoxic and release P to the overlying water (e.g., upper
PHOSPHORUS FROM INTERNAL SOURCES TABLE 2. TP release rates (mg m-1d- l ) for parts of the Great Lakes: (1) measured, (2) predicted from average lake TP concentration (Table 1, RR = 13.066xlog TP - 8.961; Niirnberg et al. 1986), and (3) predicted from sediment TP concentration (Table 1, log RR = 0.76 X log TPsed + 0.80; Niirnberg 1988). Part of Lake
(1)
(2)
(3)
Erie, West Erie, Central Erie, East Ontario, Central Bay of Quinte Niagara Basin Michigan Green Bay Superior Huron Saginaw Bay
na 7.4a na na lOb na na 26.5 c na na na
11.9 7.9 7.2 8.3 13.8 na 2.8 na -0.3 0.7 12.0
na 5.7 na 5.6 8.6 19.1 5.1 na 3.1 na na
na, no data available. aBurns and Ross 1972. bMinns 1986. CConley et al. 1987.
Bay of Quinte, western basin of Lake Erie). The computed rates predict the amount of P release in case the sediment surfaces were anoxic. It is obvious from Table 2 that the more eutrophic parts of the Great Lakes have higher expected RR. Lake Superior and Huron Lake, even if anoxic, would release only little or no P. Availability of Internally Derived P Although the quantity of P contributed from internal sources might be low compared to P supplied from external sources in the Great Lakes, the proportion potentially available to phytoplankton is typically several times higher than for P from external sources (Niirnberg and Peters 1984). Phosphorus is released from the anoxic sediments as phosphate (Niirnberg 1988), and 70-90070 stays in that biologically available form, even if diluted by surface water, as determined from anoxic hypolimnia of five Canadian Lakes (Niirnberg 1985). Besides its high availability, the timing enhances the importance of the internal load to lakes. Both upwelling of bottom water in polymictic parts and thermocline erosions in late summer in dimictic parts of the Great Lakes provide the P-deficient surface water with nutrients. Fertilization of surface water immediately after fall turnover led to increased TP concentrations and subsequently increased phytoplankton biomass in several small
135
lakes with anoxic hypolimnia (Niirnberg and Peters 1984; Peter Dillon, Ontario Ministry of the Environment, unpublished data) and some bays of the Great Lakes (Table 3, Fig. 2). It is evident from Table 3 that the increases in TP, as well as in chlorophyll concentrations, in the Bay of Quinte are less pronounced in stations closer to the open water. This is likely the result of dilution and mixing with Lake Ontario's water and means that internal load from the Bay of Quinte fertilizes the open water of Lake Ontario. This example indicates that events similar to those that occur in small, well-confined lakes may also occur in the eutrophic bays of the Great Lakes, although here the quantity of upwelling P cannot be determined easily because of the vast amount of water available for mixing. For example, in small (1,044 ha) eutrophic and nitrogen-limited Lake Magog, Quebec, which has high iron concentration in the anoxic hypolimnion, one third of the hypolimnetic P became incorporated into biomass, one third formed complexes with iron, and one third remained in the form of soluble reactive P and fertilized downstream waters (Niirnberg 1985). Elevated SRP concentrations in fall in the surface water have indeed been observed in several bays of the Great Lakes (Table 3). Restoration Possibilities In many lakes drastic cuts of external P inputs have failed to improve water quality because of the large supply of P derived from internal sources (Cullen and Forsberg 1988). To diminish the internal load of TP to any lake, either or both the AF and RR (Eq. 1) must be decreased. The decrease of the duration and extent of anoxia usually lags behind decreases in nutrients and chlorophyll concentrations (e.g., Lake Erie, Charlton 1987; Bay of Quinte, Minns and Johnson 1986; lakes with hypolimnetic withdrawal, Niirnberg 1987), possibly because the sediment oxygen demand (SOD) increases with eutrophication (Charlton and Lean 1987). Therefore, reduction of RR instead of the AF may be more successful in reducing internal P load. Rates of P release from anoxic sediment surfaces strongly depend on temperature and turbulence of the overlying water (Kamp Nielsen 1974) and P content of the surface sediment (0-10 cm, Niirnberg 1988). Of these three variables only sediment concentration can be manipulated. Total P or reductant soluble P (BD-P) in the surface sediment explained up to 87% of thes variance of anoxic RR in seven small North American lakes (Equation in
G.K.NURNBERG
136
TABLE 3. Concentrations of Tp, soluble reactive P (SRP), and chlorophyll during summer (average, sum) and after fall turnover (fall). The approximate distance (km) from the open water of Lake Ontario is given for the Bay of Quinte stations (Fig. 1). Part of Lake Bay of Quintea Hay Bay, 37 Hay Bay, 37 Picton, 28 Glenora,20 Conway, 7 Conway, 7 Erie b Saginaw Bayc
Year
TP,um
TP fall
Chlo,um
Chlo fall
SRP,um
SRP fall
78 81 78 78 78 81 82 74
25 na 18 20 19 na 13 20
82 na 82 41 22 na 21 38
10
na 8 7 3 na na na
46 na 42 36
2 14 na na 1 8 1 4
5 27 na na 3 8 7 9
11
na na na
na, no data available. aMillard 1986, Robinson 1986. bRosa 1987, Lam et al. 1987. cBierman and Dolan 1981.
Table 2; Niirnberg 1988). The regression equation predicts the RR to be zero when the P concentrations are below a background or refractory value (Le., 70.8 p,g g wet weight-I for TP, 4.8 p,g g wet weight- 1 for the reductant-soluble fraction, BD-P). The decrease of sediment P can be enhanced via hypolimnetic withdrawal, a lake restoration technique that withdraws P-rich, hypolimnetic water and thus eventually depletes the releasable fraction of sediment P (Niirnberg 1987). A faster restoration technique is the dredging of surface sediments. To prevent release completely, at least 10-15 cm of the sediments have to be dredged, because releasable P fractions may migrate upward during anoxia. With this technique, the sediment oxygen demand (SOD) will be changed to pre-eutrophication levels and, hence, both RR and AF will be decreased. Dredging has been used to reduce toxic chemicals in Hamilton Harbour, Lake Ontario. An analysis of the response of oxygen and phosphorus in the water to this operation could help evaluate the validity of dredging for reversing eutrophication. But, in general, both techniques, hypolimnetic withdrawal and dredging, are useful in small lakes only and cannot be applied to the Great Lakes to any extent. However, the natural phenomenon of P release from anoxic sediments will also decrease sediment P concentration, if not counteracted by an evergrowing input from external sources. Thus, from the sediment perspective internal P load in lakes with anoxic hypolimnia can be viewed as a selfpurification process instead of a self-fertilization
process. Although internal load still contributes to the P concentration in the water, some of it will leave the lake via the outflow, or will eventually settle on oxic hypolimnetic sediments that do not release P. The P concentration of the sediments actively releasing P (Le., anoxic sediment surfaces) will decline and the sediments will have lower RR in the future. The self-purification process will happen faster the more P becomes released, compared to that portion of the external P input that settles to the anoxic sediments. The external load at which the flux downward from external sources matches the flux upward from anoxic sediments can be considered a "threshold load." Thus, L,h,e,
* R = RR * d, g
(2)
where L,h,e, is the threshold external load (mg m- 2 yr 1), Rg is the gross retention (Le., P retention of stratified lakes without P return from internal sources). The gross retention is predicted from R g = 15/(18 + q,) as developed by Niirnberg 1984, where q, is the annual water load (m yr 1); d is anoxic days per year. Since most of the settling phosphorus ends up on the hypolimnetic sediment through sediment focusing (Davis 1973), the flux downward is multiplied by a factor SF computed from the ratio of the lake surface area (Ao) to the hypolimnetic area (A hypo ), or Lth,e,
* R * SF g
=
RR
*d
(3)
PHOSPHORUS FROM INTERNAL SOURCES
137
~'
............ 'OD
.. ;:j,
100
~ ...c: 0-4
0 ~ 0
10 :is C)
I •
1'----1
/6 '-----'---
--'-----------'
10
100
1000
Total Phosphorus (ug/L) FIG. 2. TP and chlorophyll concentrations of some bays of the Great Lakes (circles, Table 3), smaller lakes from Eastern Canada (squares), and smaller European lakes (triangles) that have anoxic hypolimnia during summer stratification. Filled symbols signifY average summer data, open symbols signifY fall turnover data. The changes are represented as short lines. The regression line represents Dillon and Rigler's (1974) regression (log Chlorophyll = 1.45 * log P - 1.14) for small North American lakes.
with
Using the conversion from RR to can also be expressed as
Lint
(Eq 1), Eq (5)
(4) Lthres
Equation 3 can be re-arranged to determine the "threshold external load," or Lthres
= (RR * d) / (Rg * SF).
(5)
=
(Lint /
Rg )
* d / (AF * SF),
(6)
or (7)
138
G.K.NURNBERG TABLE 4. External "threshold load" (Lthres' Eq 4, mg m-2yr l ) determined from release rates (RR, mg m-2d- I ), days of anoxia (d), gross Pretention (Rg = 15/(18+qs), Niirnberg 1984) and sediment focussing factor (SF, EQ 4). Lthres is also expressed as percentage (%) ofpresent external P load (L ext , mg m-2yr l ).
Lake RR d q, Rg a Bay of Quinte 36 24.25 10.0 0.36 45 Erie, Central 7.4 7.04 0.60 Muskoka Bayb 5.3 80 4.82 0.66 Chub 110 1.5 4.40 0.67 aguessed from oxygen profiles (Minns and Johnson 1986). bformer Gravenhurst Bay of Lake Muskoka.
where a anoxIC. is the sediment area overlain by anoxic water (m 2). Lth,e, of the lake stays constant only as long as it is equal to the present external load. If Lth,e, is lower than Lext , it becomes larger with time, since the sediment P concentration increases, and so does, consequently, sediment release and possibly the period of anoxia. On the other hand, if Lth,e, is higher than Lext , it becomes lower until it reaches an equilibrium with Lext . Any further decreases in Lext will lead to decreases in Lth,e, down to zero Lext and Lt hres'. at this ideal state, sediment P release ceases because of permanent oxygenation and/or zero RR. Therefore, reversal of eutrophication can only take place as long as L ext stays below Lth,e,; otherwise the process comes to a halt when sediment input and outflux are at equilibrium (Le., Lt hres = Lext). This concept also means that if the existing external load is higher than the threshold load, the anoxic sediment will accumulate P, RR will increase, and the overall internal load will increase unless a reduction in anoxia occurs. The present external loads of some anoxic parts of the lower Great Lakes are two and three times larger than the computed threshold loads (Table 4). Therefore, no immediate improvement of the trophic state can be predicted in these parts of the Great Lakes, but profundal sediment P concentration and RR will increase instead. Furthermore, P from internal sources of restricted areas likely fertilizes the open water of Lake Erie, Lake Ontario, and Lake Michigan. In order to stop the increase of eutrophication, present loads have to be decreased to threshold loads, at which point sediment P concentration, RR, and Lint will remain constant. Only at loads below Lth,e, will releasable sediment P concentration, RR, and Lint actually decrease.
SF 1.4 1.2 2.0 2.0
Lth,e, 724 463 319 126
Lext 2,350 1,000 250 99
0,10
31 46 128 127
CONCLUSIONS The ideas in this paper stress the importance of processes at the water sediment interface and the changes of the upper sediment layers that occur when eutrophication proceeds. It becomes apparent from this study that future predictions concerning the P concentrations in lakes (Le., lake water) will be more accurate if not only external P sources from the watershed are considered (e.g. "critical load", Vollenweider 1968, 1976) and internal sources like anoxic sediment surfaces (e.g. Ntirnberg 1984), but the relative proportion of the P mass from both of these sources. The relative quantities determine whether the trophic state of any given lake actually will get better or worse. When the external load equals the theoretical threshold load, the amount of sedimenting externalload balances the load from internal sources. The smaller the present external load is, compared to the threshold load (Le., the more of the total P mass entering the water originates from the sediment), the faster the degree of eutrophication will decrease via loss of P from the sediment. As long as Lext stays below Lth,e" the P resources in the sediment will be depleted and less P will be released, even if the sediment surfaces are still anoxic because of the delayed reoxygenation often observed. During these processes, P from internal and external sources will contribute to the P concentration in the water according to the model TP
=
(Lex/qs) X (1 - Rg) + Lin/qs
(8)
(Ntirnberg 1984). The concept of the threshold load helps to explain the apparent lag in the response of recently eutrophied lakes and the sudden signs of eutrophication. As long as the sediment surfaces are oxic,
PHOSPHORUS FROM INTERNAL SOURCES no, or only marginal, P release occurs (review in Niirnberg 1984) despite loads larger than the threshold load. In this case only the sediment P concentration increases. When eutrophication proceeds long enough and morphometry and hydrology are favorable (Le., low depth and slow flushing), anoxic conditions develop in the hypolimnion and the release of P from the sediment can be substantial, depending on the previous fertilization of the sediments. When an amelioration of the trophic state is attempted, external load has to be decreased below the theoretical threshold load. These concepts may also apply to other compounds that interchange frequently between the sediment and water, such as heavy metals and organic compounds.
ACKNOWLEDGMENTS I appreciate Peter Dillon's and Bruce LaZerte's encouragement and fruitful discussions and Roger Bachmann's and Thomas Young's helpful criticisms of this manuscript.
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