Inter-annual variation in submerged macrophyte community biomass and distribution: the influence of temperature and lake morphometry

Abstract

We monitored the biomass and distribution of submerged macrophyte communities in five lakes in the Eastern Townships of Quebec, Canada, in two climatically different growing seasons. Early season warm temperatures allowed for much deeper macrophyte colonization (25–170%), an average of 300 g m⁻² greater wet weight biomass (g m⁻²) and a 45–1160% increase in whole lake biomass in warm 1998 compared to a cooler 1997. In contrast, the inter-year variation in underwater light climate had no effect on macrophyte colonization as water column turbidity was generally higher in the warmer year, although irradiance did affect the distribution and biomass within seasons. The positive relationship between phytoplankton and whole system macrophyte biomass observed in this study cautions that the negative relationship in highly eutrophic shallow systems does not appear applicable to deeper systems characterized by lower macrophyte biomass to lake volume ratios. Earlier growing seasons, as predicted by climate change models, would result in greater biomass and distribution of submerged macrophyte communities, thereby modifying the structure and functioning of north temperate lakes. The effect will be a function of lake morphometry and most pronounced in shallow systems.

Introduction

Submerged macrophyte communities influence many aspects of lake structure and function. Their physical structure determines fish, zooplankton, and benthos habitat in the littoral zones of lakes, influencing food web structure (Diehl and Kornijow, 1998, Jeppesen et al., 1998, Beklioglu and Moss, 1996, Cyr and Downing, 1988). The plant beds serve as long term sinks for organic material (Benoy and Kalff, 1999), and as short term sources of nutrients and metals to the water (Landers, 1982, Jackson et al., 1994). At the whole lake scale, the pool of organic carbon represented by macrophyte communities tends to be large in shallow lakes (Wetzel and Hough, 1973). Submerged plants therefore play an important role in carbon and nutrient flux within the predominantly shallow lakes that dominate the global landscape. Macrophytes also affect the use of lakes as recreational areas. Whereas swimmers might not appreciate macrophyte beds, most sport fishers look favourably upon extensive macrophyte beds. Ecological and lake management aspects of limnology are thus affected by changes in the distribution, specific biomass (weight per unit area) and the whole lake biomass of macrophytes.

Factors that influence the biomass and distribution of submerged macrophytes both among and within lakes have been well studied. Within lakes, the morphometric variables of sediment slope and community exposure are both negatively correlated to macrophyte biomass (Duarte and Kalff, 1986). Among lakes, macrophyte biomass and distribution varies with lake morphometry (Duarte et al., 1986), latitude (Duarte and Kalff, 1987), water transparency (Chambers and Kalff, 1985, Dale, 1986) and sediment characteristics (Barko et al., 1988, Barko et al., 1991, Anderson and Kalff, 1988). The majority of studies on the distribution and biomass of macrophytes focus on factors that vary in space within growing seasons. It is, however, unclear how such environmental factors influence macrophytes on longer temporal scales.

Many lines of evidence point to the influence that light levels have on macrophytes at many different scales. In his extensive review of macrophyte distribution within lakes, Spence (1982) reports an inverse relationship between the maximum depth of macrophyte colonization and the minimum vertical diffuse attenuation coefficient among systems. He concludes that the light regime is the principal determinant of macrophyte distribution within most lakes, while other factors (temperature, hydrostatic pressure, substrate conditions) have minor effects. In their comparative study, Chambers and Kalff (1985) produce strong empirical relationships between Secchi depth and maximum depth of colonization for angiosperms, bryophytes and charophytes. Additionally, experimental work demonstrates the limitation of submerged macrophyte growth at light levels comparable to those observed at the maximum depth of colonization in lacustrine and marine environments (Sand-Jensen and Madsen, 1991, Markager and Sand-Jensen, 1992). Longer term studies link eutrophication, and associated lower light levels, with decreases in macrophyte abundance (Rørslett et al., 1986, Blindlow, 1992).

Virtually no attention has been given to the impact of environmental factors, such as temperature, which operate on inter-annual time scales (Royle and King, 1991). There is, however, a body of evidence which indicates that inter-annual fluctuations in temperature should have an effect on the distribution and biomass of macrophyte communities. Most physiological processes have temperature dependent steps, leaving rates subject to the influence of temperature. Laboratory manipulations show interactions between light and temperature in determining shoot biomass, root biomass, and shoot density in macrophytes (Barko et al., 1982). Other studies show submerged macrophytes to have a Q₁₀ ranging from 2.3 to 3.5 (Madsen and Brix, 1997). Further, plants acclimated to high temperatures have consistently higher photosynthetic capacities and light use efficiencies. The increased light use efficiency should allow plants growing in warmer water columns to grow deeper and cover larger lake bottom areas at a given irradiance. Unfortunately, a temperature effect is difficult to demonstrate in macrophyte beds where irradiance and temperature typically co-vary over the growing season, and where seasonal temperature changes tend to be moderate.

Studies on power plant cooling ponds and reservoirs allow a separation between the effects of temperature and irradiance on macrophyte communities. Grace and Tilley (1976) report a doubling of standing crop (mostly due to increased Myriophyllum spicatum) in the warm portions of a North Carolina cooling pond. In an Alberta, Canada lake, plant growth starts 2–3 months earlier in heated regions that receive cooling water from a coal fired electrical generating station (Haag and Gorham, 1977). Other factors such as maximum biomass, time of flowering, and seed production are also affected by temperature in cooling ponds (Haag, 1983).

A third line of evidence for the influence of temperature on macrophytes comes from an analysis of the macrophyte biomass and distribution among lakes in different geographical areas. Empirical and conceptual models developed by Duarte and Kalff (1987) show that temperature should influence the extent of macrophyte colonization. Changes in transparency in warmer low latitude lakes are predicted to result in greater changes in the extent of macrophyte colonization than similar light climate changes at higher latitudes. The work, however, is not designed to allow a separation of temperature and light (day length) effects. Consequently, while there is physiological, small scale, and geographical evidence for the influence of temperature on macrophyte colonization within lakes, work at the whole lake scale has been lacking.

Although the influence of inter-annual changes in temperature on macrophytes has not been documented in lakes, a 2 year marine study shows that Eelgrass communities exhibit higher biomass and production in a warm El niño year, despite a decrease in nutrient availability (Nelson, 1997). If temperature influences lacustrine macrophyte distribution above and beyond that produced by irradiance, the effect of such changes will be greatest in shallow lakes which constitute the majority of lakes worldwide. For a particular transparency, shallow lakes have a greater proportion of their sediment surface available for colonization than their deeper counterparts.

The importance of the effect of temperature on macrophyte community ecology is accentuated by a predicted increase in the temperature of the northern hemisphere resulting from an expected anthropogenically induced climate change. Predictive models are needed to address this issue. This paper has two different but linked objectives towards such models. The first is an analysis of the impact of temperature and light on the distribution and biomass of submerged macrophytes in five lakes during two quite different growing seasons. The second, and smaller objective, attempts to separate the influences of temperature and irradiance on macrophyte community distribution and biomass in the same and other north temperate lakes, using previously published data.