Selected Features of the Distribution of Chlorophyll along the Southern Shore of Lake Superior

J. Great Lakes Res. 30 (Supplement 1):269–284 Internat. Assoc. Great Lakes Res., 2004

Selected Features of the Distribution of Chlorophyll along the Southern Shore of Lake Superior Martin T. Auer* and Laura A. Bub Department of Civil & Environmental Engineering Michigan Technological University 1400 Townsend Drive Houghton, Michigan 49931 ABSTRACT. Monitoring was conducted over the April to October interval of 1999 and 2000, seeking signals in the spatiotemporal structure of algal biomass (chlorophyll) in Lake Superior. Sampling was conducted along three nearshore-offshore transects in the vicinity of Michigan’s Keweenaw Peninsula. Levels of algal biomass in Lake Superior are the lowest in the Great Lakes, with chlorophyll concentrations ranging from ~0.25 to 2.5 mg·m–3 and averaging less than 1 mg·m–3 in the surface waters. Several spatial signals were observed, including inter-transect differences, nearshore-offshore gradients, and vertical structure. These signals are thought to be mediated by several factors, including runoff from a major tributary, light availability and mixing depth, inter-transect variations in bathymetry and temperature gradients, e.g., the thermal bar in spring and vertical stratification in summer. Seasonally, surface water chlorophyll dynamics were characterized by an increase from late-winter concentrations in late April and early May, a continued increase in the nearshore and a decrease/stabilization at offshore sites from late May through July, a summer minimum in late July and August and an increase in September and October with the approach to turnover. These signals are striking given the modest levels of chlorophyll present in the lake and considering prevailing conditions of light, temperature and phosphorus availability. The spatial and temporal structure evidenced here likely resonates through the system, impacting the distribution of organisms both higher and lower in the food web. These results will find application in this and other similar systems for consideration of the vernal thermal bar as a factor mediating primary production and cross-margin transport of materials of biogeochemical significance, in developing a greater appreciation of the role of the microbial loop in fostering secondary production, in defining the relative roles of allochthonous and autochthonous contributions to the carbon budget and in supporting remote sensing studies of large scale transport phenomena in the lake. INDEX WORDS:

Chlorophyll, phytoplankton, Lake Superior, Great Lakes.

INTRODUCTION Phytoplankton production is an important source of the carbon which fuels lacustrine systems (Wetzel 2001). Levels of algal biomass and their spatiotemporal dynamics can influence the abundance and distribution of organisms both higher and lower in the food web. In Lake Superior, an understanding of the phytoplankton community is fundamental to such diverse issues as the characterization of net heterotrophy (Urban et al. 2004a) and the management of fisheries resources within the bounds of the system’s biological productivity (Horns et al. 2003). *Corresponding

Whether we are interested in phytoplankton community dynamics from a purely ecological perspective or from a desire to effectively manage the higher levels of the food web, we require a capability to quantitatively consider cause and effect, to test the relationship between stimulus and response as a reflection of our understanding of ecosystem structure and function. This capability is often realized through the application of mathematical models that accommodate the role of nutrients, environmental conditions (light and temperature), and loss processes (settling, grazing) in mediating the distribution of phytoplankton in time and space (Chapra 1997). The modeling process calls upon us to formalize our vision of stimulus-response by developing algorithms representing key physiological

author. E-mail: [email protected]

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processes, e.g., growth as a function of nutrient availability. Model predictions can then be compared with field observations in an effort to gain confidence in our understanding of the cause-effect relationship. In this sense, field observations serve a twofold function: first, in providing an image of system behavior which helps to form and guide our concept of stimulus and response and second, in offering a target against which we can verify the accuracy of that vision. Here we describe spatial and temporal signals in phytoplankton standing crop (chlorophyll) at a site on Lake Superior, suggesting relationships with environmental forcing conditions that may govern those distributions. This effort is intended to set the stage for subsequent exercises leading to development of the predictive capacity desired by audiences in both pure and applied limnology. OBJECTIVES AND STUDY SITE Lake Superior is the largest, deepest and coldest of the Laurentian Great Lakes. Soluble phosphorus concentrations often approach the level of analytical detection (Baehr and McManus 2003) and attendant conditions of oligotrophy are reflected in low phytoplankton biomass (Barbiero and Tuchman 2001a) and high water clarity (Schertzer et al. 1978). Lake Superior lacks the strong gradients in algal standing crop evident at other locations in the Great Lakes (Lake Erie, Makarewicz et al. 2000; Green Bay, Auer and Canale 1986; and Saginaw Bay, Effler 1984) and the nutrient dynamics observed in Lakes Michigan (nitrate and silica depletion, Bartone and Schelske 1982, Schelske 1985) and Erie (nitrate depletion, Makarewicz et al. 2000). Given its low levels of algal standing crop, one might conclude that spatiotemporal signals in chlorophyll, i.e. the footprints of the phytoplankton community, are rare in Lake Superior. That conclusion would be incorrect. Putnam and Olson (1966) were among the first to characterize conditions of algal standing crop in Lake Superior, reporting summer surface water chlorophyll concentrations of 0.4–1.4 mg·m–3. That study, and a companion investigation (Olson and Odlaug 1966), provided early documentation of the deep chlorophyll maximum (peak concentrations of 2–3 mg·m –3). Subsequent work by Watson et al. (1975) described a chlorophyll maximum (again with concentrations of 2–3 mg·m –3) located just

below the thermocline and extending horizontally over large areas of the lake. The landmark survey conducted by the Great Lakes Biolimnology Laboratory (Canada Centre for Inland Waters) between May and November of 1973 led to a comprehensive description of phytoplankton biomass and composition in Lake Superior (El-Shaarawi and Munawar 1978, Munawar and Munawar 1978, Munawar et al. 1978). The phytoplankton assemblage, composed of 285 taxa, was reported to be dominated by phytoflagellates and diatoms (Munawar and Munawar 1978) with nannoplankton (< 64 µm) being the most important contributors to both species composition and biomass (Munawar et al. 1978). Surface water chlorophyll concentrations ranged from ~0.4–2.0 mg·m–3 (El-Shaarawi and Munawar 1978) with phytoplankton biomass homogenously distributed across the lake. No nearshore-offshore gradients were observed (Munawar and Munawar 1978). Seasonal fluctuations were small by comparison to other, more productive systems, but a summer peak in chlorophyll was noted (Munawar and Munawar 1978). Sub-surface maxima in phytoplankton biomass were observed at various depths on several dates during this study. More recently (1983–present), U.S. EPA has been conducting surveillance monitoring of the offshore waters of the Great Lakes, including Lake Superior. Phytoplankton biomass was reported to be substantially lower in Lake Superior than in the other lakes, with summer average chlorophyll concentrations of < 0.5 mg·m –3. The Lake Superior phytoplankton assemblage displayed the most diversity of the lakes (~170 taxa), dominated in biomass by cryptophytes and diatoms (Barbiero and Tuchman 2001a). Results from the surveillance program have also served to substantially enhance documentation of the deep chlorophyll maximum in Lake Superior (Barbiero and Tuchman 2001b, Barbiero and Tuchman 2004). Despite these and other contributions, phytoplankton dynamics in Lake Superior remain the least studied and, arguably, the least well described for any of the Laurentian Great Lakes. This is likely a result of the lake’s size and remote location and perhaps because of the perception that unperturbed, oligotrophic systems merit less attention. Resource constraints force those exploring lower trophic level dynamics in the lake to make tradeoffs in spatial and temporal resolution. The result is an emerging, but incomplete, picture of the dynamics of the phytoplankton community in Lake Superior: a pic-

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ture where signals are indeed present and which rival those of the other Great Lakes by virtue of the extreme conditions under which they are manifested. In this manuscript, we describe spatiotemporal dynamics in phytoplankton standing crop derived from measurements made in 1999 and 2000 along three transects on the south shore of Lake Superior adjacent to Michigan’s Keweenaw Peninsula. The transects were spaced along 110 km of lakeshore and encompass environments with widely differing bathymetries. The field years include pre-stratification, stratified, and post-stratification periods and their attendant light and temperature regimes. The results presented support several studies on Lake Superior, including efforts relating to bacterioplankton dynamics (Auer and Powell 2004), carbon cycling (Urban et al. 2004a) and remote sensing (Budd 2004, Budd and Warrington 2004, Gons and Auer 2004, and Li et al. 2004). We believe that this pool of observations will serve well in stimulating further consideration of cause-effect relationships in the lake’s phytoplankton ecology and will provide a vehicle for testing the performance of the conceptual and mathematical models that will lead to a more fundamental understanding of ecosystem function. METHODS Sampling A monitoring program was conducted during the ice-free seasons of 1999 (six dates, May–October) and 2000 (seven dates, Apr–October) along three onshore-offshore transects originating near Ontonagon (ON), Houghton (HN), and Eagle Harbor (EH), Michigan (Fig. 1a). The three transects offer a range of bathymetries, with ON exhibiting shallower depths and overall warmer temperatures, EH having a steeper depth gradient and colder temperatures and HN being intermediate in both respects (Fig. 1b). The offshore extent of sampling and the number of stations sampled varied among transects to accommodate gradients in bathymetry. Stations along the HN transect (Fig. 1c) were grouped by depth into nearshore (10 –15 m), mid-transect (70–125 m) and offshore (150–180 m) regions for analysis of spatial and seasonal trends. Only midtransect and offshore sites exhibit thermal stratification. Full depth profiles were collected in the offshore region of the HN transect, while measurements in the nearshore region and on the ON and EH transects were limited to surface waters.

FIG. 1. Description of the study site on Lake Superior, including: (a) transect locations, (b) transect bathymetry, and (c) station locations along the HN transect. Sample Collection and Processing, and Data Analysis Surface water samples were collected with a clean plastic bucket and samples at depth were collected with 5- and 30-L Niskin bottles. Triplicate samples for chlorophyll analysis were filtered immediately following collection (1-L volume, glass fiber filter, MgCO3 addition). Filters were wrapped in aluminum foil, frozen and stored pending analysis. Chlorophyll was analyzed using the method of Parsons et al. (1984). Filters were extracted in 90% acetone, and the fluorescence of the extract measured using a Shimadzu RF-1501 spectrofluorometer. The instrument was calibrated using pure chlorophyll extract (APHA 1998). Vertical temperature and chlorophyll profiles were generated using a

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Seabird Electronics CTD (SBE-25 CTD) equipped with a WETStar fluorometer (WET Labs). The fluorescence detection (chlorophyll) capability of the profiler was calibrated against spectrofluorometer measurements of samples collected at several depths. Calibrations were performed for each transect of each cruise to accommodate shifts in the profiler response. Averages are presented as the mean ± standard deviation. Statistical differences are evaluated using a two-tailed, Student’s t-Test with significance indicated when p < 0.05. RESULTS AND DISCUSSION The chlorophyll database, obtained over the April–October interval of 1999 and 2000, was examined for the presence of signals in spatial (intertransect, nearshore-offshore and vertical) and temporal structure. Spatial Signals Inter-Transect Differences The largest spatial scale addressed here was that of local or inter-transect variability. Differences in algal standing crop among transects can reflect the impact of a point source of nutrients, local conditions of bathymetry and thermal structure, and/or regional patterns in water quality. For example, Barbiero and Tuchman (2001a) reported a high degree of variability in phytoplankton biomass both among (regional) and within (local) the basins of Lake Erie. Much less variation is expected for Lake Superior, as phytoplankton biomass is known to be more homogenously distributed (Munawar and Munawar 1978, Barbiero and Tuchman 2001a). However, conditions do exist at the study site which may result in inter-transect differences in algal biomass. Surface water chlorophyll concentrations averaged 1.0 ± 0.6, 0.7 ± 0.3, and 0.6 ± 0.2 mg·m–3 along the ON, HN, and EH transects, respectively, in 1999. Concentrations for the ON transect were significantly higher than those for the HN and EH transects (p < 0.005) and levels on the HN transect was significantly greater than those for the EH transect (p < 0.02). In 2000, surface water chlorophyll concentrations averaged 0.8 ± 0.3, 0.8 ± 0.3, and 0.7 ± 0.3 mg·m–3 along the ON, HN, and EH transects, respectively. Concentrations for the ON (p < 0.10) and HN (p < 0.05) transects were significantly higher than those for the EH transect, while

FIG. 2. Relation of chlorophyll dynamics to the timing of the spring runoff event and thermal bar formation in (a) 1999 and (b) 2000. those of the ON and HN transects were not significantly different. Two signals are apparent in these results. The first relates to the observation that mean chlorophyll levels along the ON transect were significantly higher than those of the HN transect in 1999, but not in 2000. The ON transect lies proximate to the Ontonagon River, one of the largest sources of terrigenous input to this region of the lake (Auer and Gatzke 2004). Rivers such as this have the capacity to markedly enrich local environments, particularly during the spring runoff event when tributaries in the study region deliver ~70% of their annual load (Auer and Gatzke 2004). The statistical significance of the difference between the ON and HN transects in 1999 is due solely to elevated chlorophyll concentrations observed along the ON transect on 11 and 19 May (Fig. 2a). These dates lie within the period influenced by the spring runoff event (Fig. 2a; Auer and Gatzke 2004). The timing of the signal and the location of the sampling sites relative to the Ontonagon River suggest that this inter-transect difference may be driven by tributary inputs of nutrients. Chlorophyll concentrations along the ON transect in late April and early May of 2000 were elevated, but to a much lesser extent than in 1999, and the

Chlorophyll in Lake Superior difference in mean concentrations between the ON and HN transects was not statistically significant. The 2000 spring runoff event was smaller (~onehalf the discharge of 1999) and shorter in duration (3 weeks less than in 1999). Further, the May 1999 sampling took place during the runoff event (Fig. 2a), whereas the river had been back at base flows for three weeks prior to the May 2000 sampling (Fig. 2b). Thus, the timing and magnitude of the 2000 spring runoff event may have been such that its impact on algal standing crop was not captured in our sampling. As a result, no significant difference in mean chlorophyll concentrations was noted between the ON and HN transects that year. Given the contrasting bathymetries of the transects (Fig. 1b), it is worthwhile to consider the potential for inter-transect variation in temperature as a factor underlying the differences in chlorophyll concentrations observed between the ON and HN transects. Inspection of satellite images for Lake Superior (AVHRR, J.R. Budd, Great Lakes Imagery Archives) indicates that nearshore waters warmed at approximately the same rate in the vicinity of the ON and HN transects and thus inter-transect differences in temperature are unlikely candidates as causal factors. Further, a slight delay in warming in 2000 compared with 1999 (onset of thermal bar, Figs. 2a and b) had no apparent effect on the temporal structure of the chlorophyll peaks. The second signal apparent from these results relates to the observation that, in both years, mean surface chlorophyll levels along the ON (p < 0.002 in 1999, p < 0.1 in 2000) and HN (p < 0.02 in 1999, p < 0.05 in 2000) transects were significantly higher than along the EH transect. This signal may be a reflection of bathymetric differences between the relatively shallow waters at ON and HN versus those at EH, where depths > 100 m are encountered within 0.5 kilometers of shore. In deeper waters, nearshore warming in spring is slowed and cold temperatures and low average light (due to increased mixing depth) are experienced until the onset of stratification. These conditions, unfavorable to phytoplankton growth, may be responsible for the observed differences between the ON/HN and EH transects. In summary, inter-transect signals in chlorophyll concentration were observed in apparent response to (1) proximity of a major source of terrigenous input, (2) bathymetric mediation of the light and temperature environment, and (3) simple dilution of the production over a larger mixed depth. These subtle signals offer evidence of interactions be-

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tween the phytoplankton community and their environment heretofore unrecognized in this oligotrophic system. In some respects, it is surprising that such signals would be sustained given the high degree of mixing characteristic of large lakes (Chapra 1997) and the potential for mixing and mass transport associated with the Keweenaw current (Viekman and Wimbush 1993). The work of Budd (2004) has demonstrated the value of these chlorophyll patterns in describing large scale transport phenomena in Lake Superior. Nearshore-Offshore Gradients Horizontal gradients in algal standing crop, i.e. those occurring with increasing distance offshore, are one of the most widely recognized spatial signals evidenced in the Great Lakes phytoplankton community. Studies of Lakes Michigan (Moll et al. 1993a, 1993b; Stoermer 1968) and Ontario (Nalewajko 1967) have shown that algal standing crop is higher and species diversity greater in nearshore than in offshore environments. These differences are most often ascribed to favorable conditions of nutrient supply and temperature in the nearshore, particularly as associated with the development and migration of the vernal thermal bar. Others have suggested that growth is regulated by the mixed layer depth (Sverdrup 1953, Fahnenstiel et al. 2000), with light limitation in effect at various times in Lake Michigan (Fahnenstiel et al. 2000) and Lake Superior in deeper offshore waters (Nalewajko and Voltolina 1986, Schulz et al. 2001). The formation and relaxation of horizontal gradients in surface water chlorophyll in Lake Superior were tracked in 1999 and 2000 along the HN transect. Early in the 1999 field year (7 and 17 May), surface water chlorophyll concentrations along the HN transect ranged from 0.6 to 1.0 mg·m–3 with little apparent difference among nearshore, mid-transect and offshore sites (Fig. 3). Biomass levels were somewhat depressed, however, at the nearshore stations on 17 May. A gradient was first observed on 4 June, with nearshore concentrations 50% higher than those of mid-transect and offshore waters. The gradient steepened through the next two sampling dates (18 June and 2 July), with maximum concentrations in the nearshore reaching 1.5 mg·m–3, and remained evident through two subsequent samplings (14 and 28 July). The signal then relaxed, leading to homogeneity on 9 and 24 August with transect average chlorophyll concentrations of 0.5 and 0.4 mg·m–3, respectively. Homogeneous condi-

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FIG. 4. Horizontal (nearshore-offshore) distribution of surface water chlorophyll along the HN transect for sampling dates in 2000. FIG. 3. Horizontal (nearshore-offshore) distribution of surface water chlorophyll along the HN transect for sampling dates in 1999. tions persisted through the last sampling (8 October), with transect average concentrations increasing to 0.8 mg·m–3. A similar pattern was observed in 2000. Transect average surface water chlorophyll concentrations ranged from 0.5 to 0.7 mg·m–3 for the first three sampling dates (28 April, 11 May, and 9 June) and no nearshore-offshore gradient was apparent (Fig. 4). A gradient was first observed on 22 June, with nearshore concentrations (1.3 mg·m–3 ) doubling those of offshore waters. The gradient was maintained through the end of July. Homogeneous conditions were observed over the balance of the field season, with transect average chlorophyll concentrations increasing from 0.8 mg·m–3 on 25 August to 1.1 mg·m–3 on 24 September to 1.3 mg·m–3 on 20 October. A clear and systematic signal emerges from these observations: nearshore-offshore gradients in algal standing crop occur in Lake Superior, bounded in time by periods of essentially homogenous conditions. Gradients develop in early- to mid-June, as

nearshore chlorophyll concentrations increase, and are maintained through mid- to late-July. Development of the gradient is coincident with the spring warming of nearshore waters (offshore sites remain cold) and relaxation of the gradient occurs with the onset of thermal stratification (warm surface water conditions at both nearshore and offshore sites). The gradient is striking and stands in contrast to earlier surveys of Lake Superior where nearshoreoffshore differences were not observed (Munawar and Munawar 1978). Vertical Structure—the Deep Chlorophyll Maximum Another widely recognized signal in the Great Lakes phytoplankton relates to vertical structure and is manifested in the phenomenon termed the deep chlorophyll maximum (DCM). The formation and dissipation of the subsurface maximum was tracked in 1999 and 2000 along the HN transect (Figs. 5 and 6). There was no evidence of a subsurface maximum in pre-stratification sampling (7 and 17 May 1999, 11 May and 22 June 2000) or, during stratification, at nearshore sites where the entire water column lies within the epilimnion. Development of the DCM paralleled that of thermal stratification (2 and 14 July and 24 August 1999 and 30

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FIG. 5. Vertical structure in the distribution of chlorophyll and temperature at stations along the HN transect in 1999. July and 25 August 2000). With the disruption of stratification in fall (8 October 1999, 24 September and 20 October 2000), chlorophyll concentrations within and above the metalimnion remained elevated, but the distinct peak associated with the DCM was absent. A more detailed inspection of the results follows, focusing on thermally-stratified mid-transect and offshore stations. In 1999, observations of the DCM were first made during the 2 July cruise (Fig. 5). The peak of the DCM was well defined at the deeper mid-transect stations, residing within the thermocline at a depth of 16m. At the offshore stations, the peak was less well developed, deeper (30 m), and was posi-

tioned nearer the metalimnion-hypolimnion boundary. Thermal stratification was not as strong at the offshore stations and surface temperatures were colder. By 14 July, the peak of the DCM had relaxed at the mid-transect stations and was deeper (25 m) than during the previous sampling, now residing at the metalimnion-hypolimnion boundary. The peak of the DCM at offshore stations had, by this time, strengthened and remained resident at a depth of 30 m, near the metalimnion-hypolimnion boundary. Surface water temperatures were still colder and stratification less strongly developed at the offshore stations. Stratification was fully developed at all stations by the time of the 24 August

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Vertical structure in the distribution of chlorophyll and temperature along the HN transect in

cruise. The peak of the DCM was resident at a depth of 25–30 m across the transect, again associated with the base of the metalimnion. The strength of the peaks was somewhat relaxed relative to earlier dates. By the last cruise (8 October), the thermocline had weakened markedly and no striking vertical structure in chlorophyll was evident, although concentrations were generally higher above the metalimnion-hypolimnion boundary. No sampling was conducted during the period of DCM formation in 2000. Thermal stratification was well established across the entire transect by the time of the 30 July cruise (Fig. 6) and the DCM peak was well defined, positioned at a depth of

25 m (well within the metalimnion). By 25 August, the peak of the DCM had deepened to 35 m and was located at the metalimnion-hypolimnion boundary. The epilimnion was considerably deeper during the 24 September and 20 October cruises and vertical structure in chlorophyll was largely eroded at the mid-transect stations. Chlorophyll concentrations remained elevated above the metalimnion-hypolimnion boundary at the more offshore stations, but no distinct peaks were evident. The deep chlorophyll maximum is one of the most striking signals in the phytoplankton of Lake Superior. The formation, evolution and dissipation of this subsurface peak clearly tracks that of ther-

Chlorophyll in Lake Superior mal stratification, however interannual differences in the strength (vertical chlorophyll gradient) and uniformity (character of the signal with distance offshore) were observed. It is not clear whether these differences reflect variation in temperature structure (a mass transport effect) or in the availability of chlorophyll (for growth or trapping within the metalimnion). Beyond this, there remains considerable controversy regarding the factors driving the phenomenon, here and in other systems. Changes in cellular chlorophyll content (Barbiero and Tuchman 2004) explain some, but not all (Urban et al. 2004b) of the increase in subsurface pigment levels. Two lines of reasoning are typically followed in explaining the nature of the DCM. Some believe that it reflects a favored environment, i.e., a position in the water column where light penetration remains sufficient to support photosynthesis and where nutrients are supplied by recycling from deeper waters. Others favor a hypothesis in which phytoplankton which have been circulating through a deep water column enjoy a growth pulse following the onset of stratification (smaller mixing depth) and sedimentation of that pulse is subsequently registered as a subsurface maximum as it moves through the metalimnion. It is beyond the scope of this manuscript to address these points further, however, the observations presented here may prove of value in doing so in the future. Temporal Signals Seasonality in surface water chlorophyll concentrations is examined here for nearshore and midtransect/offshore stations on the HN transect. Chlorophyll levels were very low in late-March of 1999, averaging 0.2 ± 0.01, 0.2 ± 0.01, and 0.3 ± 0.01 mg·m–3 at nearshore, mid-transect, and midlake (48°06.44′N 88°14.57′W) stations, respectively (Fig. 7a). Water temperatures were slightly above 2°C in the nearshore and slightly less than that at mid-transect and mid-lake sites. Despite continued cold conditions (2–4°C, Fig. 7b), and perhaps responding to increasing incident solar radiation and photoperiod, chlorophyll concentrations rose dramatically through April and into early May at all stations, reaching ~0.9 mg·m–3. Increases at nearshore stations preceded those at mid-transect and offshore sites. Trends in chlorophyll at nearshore and mid-transect/offshore sites differed following this initial pulse. Concentrations continued to increase at nearshore stations, tracking trends in water temperature and reaching a maxi-

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mum of 1.4 mg·m–3 in early July. Levels then decreased over the balance of the summer at nearshore stations reaching a minimum of 0.2 mg·m–3 observed in late August. Concentrations at the mid-transect and offshore stations fell through May and June, despite increases in water temperature, hovering near 0.5 mg·m–3 over the summer with a minimum of 0.4 mg·m–3 observed in late August. Surface water chlorophyll levels increased dramatically at all stations with the approach to turnover, averaging 0.8 mg·m –3 along the entire transect. Sampling was less temporally intense in 2000, however, several of the features noted in 1999 were observed here as well. Chlorophyll levels were similar at both nearshore and mid-transect/offshore stations (0.5 mg·m–3) by the time sampling was initiated in late April (Fig. 7c) and had risen above the presumed winter baseline of ~0.2 mg·m–3 (Fig. 7a). Water temperature at this time was ~3°C (Fig. 7d). Concentrations increased slightly through early May (more so at nearshore stations) before dropping through late May and into early June. Temperatures remained cold and stable over this interval: 6–7°C at nearshore and 3–4°C at midtransect/offshore stations. As in 1999, trends in chlorophyll concentration differed for nearshore and mid-transect/offshore stations after this. Concentrations in the nearshore increased dramatically, reaching a maximum of 1.3 mg·m–3 in late June, while those at mid-transect and offshore stations remained lower and more stable at ~0.6 mg·m–3. Chlorophyll levels fell to summer minima of 0.5 and 0.3 mg·m–3 at nearshore and mid-transect/offshore stations, respectively, in late July and then increased through late August, September and October to a maximum of ~1.2 mg·m–3 with the approach to turnover. The seasonal signal in surface water chlorophyll levels is characterized by four features: (1) an increase from later-winter levels which occurs across the entire transect in late April and early May, (2) a continued increase at nearshore stations and a decrease/stabilization at mid-transect/offshore sites from late May through July, (3) a summer minimum which occurs across the entire transect in late July and August, and (4) an increase which occurs across the entire transect with the approach to turnover. Environmental factors driving these patterns are discussed below as the signals are integrated.

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FIG. 7. Seasonal structure in surface water (a, c) chlorophyll and (b, d) temperature at nearshore, midtransect and offshore stations along the HN transect in 1999 and 2000.

Integrating the Signals The spatiotemporal features of chlorophyll structure in Lake Superior, examined individually above, are considered here in an integrated fashion using two-dimensional distance-depth (spatial structure in 1999, Fig. 8) and time-depth (temporal structure in 2000, Fig. 9) plots. Distance-Depth Plot, Temporal Signals The development of thermal structure and its potential for impact on chlorophyll distribution is nicely demonstrated in the distance-depth plot for 1999 (Fig. 8). Temperature and chlorophyll conditions were largely homogeneous in early May, although a slight warming and an increase in chlorophyll was evident at extreme nearshore stations. Thermal bar formation was initiated in the nearshore in mid-May and the spring chlorophyll

pulse was well developed at both nearshore and mid-transect sites. Increased levels of algal standing crop evident here in mid-May, but absent at offshore locations, may reflect the effects of light limitation (differences in mixed layer depth) as very little temperature structure is present at this time. The thermal bar was well established in early July, extending its position offshore through mid-July, and stratification was complete at the time of the August sampling. The primary signal associated with evolution of the spring and summer thermal structure is that of the deep chlorophyll maximum. A prominent subsurface maximum was present at mid-transect sites in early July, dissipating there and forming at offshore locations by mid-July. In August, the intensity of the chlorophyll gradient within the DCM was much less than in July, but the structure was present and well distributed across all mid-transect and offshore stations. Chlorophyll

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FIG. 8. Spatiotemporal distribution of chlorophyll and temperature along the HN transect as illustrated for 1999 using distance-time plots. Black lines indicate sampling station locations. Y-axis is depth, ranging from 0–160 m.

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FIG. 9. Spatiotemporal distribution of chlorophyll and temperature along the HN transect as illustrated for 2000 using depth-time plots. Black lines indicate sampling dates. Y-axis is depth, range as indicated on each panel.

Chlorophyll in Lake Superior concentrations both above and below the DCM drop markedly during the thermally stratified period. The thermal structure of the lake is disrupted in October, with the metalimnion-hypolimnion boundary retained, and waters above that mixing and cooling. The distribution of chlorophyll follows the temperature patterns with concentrations above the hypolimnion essentially homogenous and elevated with respect to those of deeper waters. Two points relating to the distance-depth plot merit further discussion. The first of these is the fact that chlorophyll levels in early May (~0.9 mg·m–3) were markedly higher across the transect and over the entire water column than during the limited sampling conducted in late March (~0.2 mg·m–3). This increase occurred over a period in which water temperatures remained quite cold (2–4°C) and was thus apparently uncoupled from the environmental forcing conditions thought to mediate other features of the seasonal chlorophyll structure, e.g., formation of the thermal bar and changes in mixing depth. It is likely that increases in solar radiation and photoperiod had a positive effect on algal growth rates. Uncertainty relating to the nature and origin of this feature of the distribution of chlorophyll reflects the difficulties of gaining access to Lake Superior during March and April. The second point involves patterns in the development of the deep chlorophyll maximum. The lakeward progression of DCM formation, followed in turn by dissipation of that structure at sites where it formed earliest, seems consistent with the idea that chlorophyll trapping at the metalimnion-hypolimnion boundary plays a role in governing the phenomenon. The idea being that increases in water temperature and reductions in the mixing depth associated with the formation of thermal stratification support a brief pulse in surface water chlorophyll concentrations. Settling of these organisms, with attendant trapping along the density gradient associated with the metalimnion, leads to formation of the DCM. With no means for nutrient re-supply to the epilimnion, growth there slows, chlorophyll concentrations drop within the DCM, and a near steady state is maintained through late summer. Time-Depth Plot, Spatial Signals Differences in the structure of the distribution of chlorophyll among nearshore, mid-transect and offshore stations are evident in the time-depth plot for 2000 (Fig. 9). A spring pulse in chlorophyll occurs

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in May and June at nearshore and mid-transect sites, but not at offshore locations. The spring pulse is followed by a period in mid-summer where chlorophyll concentrations fall markedly in the surface waters of all sites and within the hypolimnion of those locations which thermally stratify. These reductions are coincident with the formation of the deep chlorophyll maximum. Finally, chlorophyll levels increase at all locations in fall with the breakdown of thermal structure. Additional discussion focuses on two features of the time-depth plot. First, the spring pulse of chlorophyll at nearshore and mid-transect locations may be especially important to food web dynamics, potentially mediating the timing and location for delivery of autochthonous production to the benthic community. For example, Fitzgerald and Gardner (1993) have proposed that the spring diatom bloom serves as the primary source of nutrition for the amphipod Diporeia in Lake Michigan. Kahn and Auer (2004) have documented striking signals in the distribution of this benthic invertebrate across the coastal margin of Lake Superior. Locations with the greatest abundance of amphipods include nearshore and mid-transect sites which experience a spring chlorophyll pulse. Offshore locations, not associated with this May and June increase in chlorophyll support markedly lesser numbers of Diporeia. The second feature deals with increases in surface water chlorophyll levels in fall as the thermal structure deteriorates. This phenomenon has been noted in Lake Superior by Putnam and Olson (1966) where chlorophyll concentrations over the upper 5 meters increased from 0.7 mg·m -3 on 30 August to 0.9 mg·m-3 on 7 September to 1.6 mg·m3 on 15 September. Urban et al. (2004b) observed similar patterns in transmissivity in the fall in Lake Superior, likely related to increases in algal biomass. This fall increase may be associated with improved environmental conditions, e.g., recycling of nutrients associated with disruption of the metalimnion, or may simply reflect re-distribution of phytoplankton from the DCM over the water column above the metalimnion-hypolimnion boundary. Absent some of the threats and pressures associated with more highly developed basins in the Great Lakes, management interest in Lake Superior finds focus in the maintenance of a sustainable fishery (Horns et al. 2003). The ability to manage that fishery with appropriate attention to the effects of nutrient availability, climate change and alterations to the food web (both planned and unplanned species introductions) requires an ecosystem approach

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(Great Lakes Fishery Commission 2001) which recognizes the importance of primary production and energy transfer in sustaining levels of prey (Flint 1986). An understanding of spatiotemporal dynamics in the phytoplankton is implicit in that ecosystem perspective. Here we have presented a picture of algal standing crop in Lake Superior which stands in contrast to that typically envisioned, i.e., microgram-per-liter concentrations extending indefinitely in time and space. The structure in the phytoplankton community now apparent suggests a sensitivity to nutrient inputs, a responsiveness to physical conditions and a variability in rates of delivery across the food web which must be accommodated in ecosystem management efforts. It remains to build on this description of spatiotemporal dynamics to advance our understanding of carbon flux in Lake Superior and to develop predictive capabilities for simulating the relationship between environmental variables and the production that will sustain the ecosystem. SUMMARY AND CONCLUSIONS Chlorophyll concentrations in Lake Superior were monitored along three transects extending lakeward from Michigan’s Keweenaw Peninsula. Sampling was conducted over the May to October interval of 1999 and the April to October interval of 2000. Considerable spatiotemporal structure in the distribution of algal biomass, as manifested in chlorophyll concentration, was noted. Chief among these signals were: (1) inter-transect differences likely related to inputs from the Ontonagon River and to variability in bathymetry, (2) nearshore-offshore gradients, forming as waters warm in spring and dissipating with the onset of thermal stratification, (3) the presence of a deep chlorophyll maximum, associated with the development of vertical temperature structure and persisting until the deterioration of stratification in fall and (4) a seasonal cycle which includes a spring pulse, a mid-summer minimum and a fall increase in pigment concentrations. While it is clear that the lake’s temperature regime, e.g., the vernal thermal bar, vertical stratification and destratification, plays a major role in mediating these signals, other forcing conditions such as tributary discharges, bathymetry/mixing depth, and light availability may be important as well. Elucidation of factors contributing to observed increases in algal biomass in March and April is hindered by limitations to lake access at that time of year. The chlorophyll signals evidenced

here may provide a basis for further explorations of the environmental conditions mediating algal dynamics in Lake Superior, especially in providing a database for calibration and verification of nutrientphytoplankton models. ACKNOWLEDGMENTS Comments offered by Joe DePinto, Noel Urban, and an anonymous reviewer improved the quality of the manuscript and are deeply appreciated. The authors would like to thank the captains and crew of the R/V Laurentian and the R/V Blue Heron for support of field operations. This paper is a contribution of the Keweenaw Interdisciplinary Transport Experiment in Superior (KITES Project) funded by the National Science Foundation under Grant No. OCE-9712872. REFERENCES APHA (American Public Health Association). 1998. Standard Methods for the Examination of Water and Wastewater, 20th Edition. Washington, D.C.: American Public Health Association, American Water Works Association, and Water Environment Federation. Auer, M.T., and Canale, R.P. 1986. Mathematical modeling of primary production in Green Bay (Lake Michigan, USA): A phosphorus and light-limited system. Hydrobiological Bulletin 20(2):195–211. ———, and Powell, K.D. 2004. Heterotrophic bacterioplankton dynamics at a site off the southern shore of Lake Superior. J. Great Lakes Res. 30 (Suppl.1): 214–229. ———, and Gatzke, T.M. 2004. The spring runoff event, thermal bar formation, and cross margin transport in Lake Superior. J. Great Lakes Res. 30 (Suppl. 1): 64–81. Auer, N.A., and Kahn, J.E. 2004. Abundance and distribution of benthic invertebrates, with emphasis on Diporeia, along the Keweenaw Peninsula, Lake Superior. J. Great Lakes Res. 30 (Suppl. 1):340–359. Baehr, M.M., and McManus, J. 2003. The measurement of phosphorus and its spatial and temporal variability in the western arm of Lake Superior. J. Great Lakes Res. 29(3):479–487. Barbiero, R.P., and Tuchman, M.L. 2001a. Results from the U.S. EPA’s Biological Open Water Surveillance Program of the Laurentian Great Lakes: I. Introduction and phytoplankton results. J. Great Lakes Res. 27(2):134–154. ———, and Tuchman, M.L. 2001b. Results from the U.S. EPA’s Biological Open Water Surveillance Program of the Laurentian Great Lakes: II. Deep chlorophyll maxima. J. Great Lakes Res. 27(2):155–166.

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