**3.3 Hydrodynamic versus plankton**

Hydrodynamic processes and biological changes occurred over different spatial and temporal scales in these two large and long subtropical lakes. Itapeva Lake (31 km long) is almost one-third the size of Lake Mangueira (90 km long), and therefore the hydrodynamic response is faster in Itapeva Lake. On a time scale of hours, we can see the water movement from one end of the lake to the other (e.g., from N to S during a NE wind and in the opposite direction during a SW wind). Because of this rapid response, the plankton communities showed correspondingly rapid changes in composition and abundance, especially the phytoplankton when the resources (light and nutrients) responded promptly to wind action. This interaction between wind on a daily scale (hours) and the shape of Itapeva Lake was a determining factor for the observed fluctuations in the rates of change for phytoplankton (Cardoso & Motta Marques, 2003) as well as for the spatial distribution of plankton

Hydrodynamic Control of Plankton Spatial and

changes in Itapeva Lake.

1990; Jørgensen et al., 2008).

**4. Conclusion** 

Temporal Heterogeneity in Subtropical Shallow Lakes 43

changed quickly. Depending on factors such as fetch and wind, areas dominated by downwelling and upwelling could be identified in the deepest parts. We observed a significant horizontal spatial heterogeneity of phytoplankton associated with hydrodynamic patterns from the south to the north shore (littoral-pelagic-littoral zones) over the winter and summer periods. Our results suggest that there are significant horizontal gradients in many variables during the entire year. In general, the simulated depth-averaged chlorophyll-a concentration increased from the pelagic to the littoral zones. This indicated that a higher zooplankton biomass can exist in the littoral zones, leading, eventually, to

Moreover, as expected for a wind-exposed shallow lake, Lake Mangueira did not show marked vertical gradients. The field campaigns showed that the lake is practically unstratified, emphasizing the shallowness and vertical mixing caused by the wind-driven hydrodynamics. This complete vertical mixing, as expected, was noted for both the pelagic and littoral zones. However, we are still of the opinion that incorporation of horizontal spatially explicit processes associated with the hydrodynamics is essential to understand the dynamics of a large shallow lake. The occurrence of hydrodynamic phenomena such as the seiches between the extreme ends, in a very long and narrow lake, is important, since seiches function as a conveyor belt, accounting for the vertical mixing and transportation of materials between the two ends of the lake and between the wetlands in the North and South areas in Lake Mangueira. Seiches was very important to explain much of the spatial

Recognition of the importance of spatial and temporal scales is a relatively recent issue in ecological research on aquatic food webs (Bertolo et al., 1999; Woodward & Hildrew, 2002; Bell et al., 2003; Mehner et al., 2005). Among other things, the observational or analytical resolution necessary for identifying spatial and temporal heterogeneity in the distributions of populations is an important issue (Dungan et al., 2002). Most ecological systems exhibit heterogeneity and patchiness over a broad range of scales, and this patchiness is fundamental to population dynamics, community organization and stability. Therefore, ecological investigations require an explicit determination of spatial scales (Levin, 1992; Hölker & Breckling, 2002), and it is essential to incorporate spatial heterogeneity into ecological models to improve understanding of ecological processes and patterns (Hastings,

Water movement in aquatic systems is a key factor which drives resources distribution, resuspend and carries particles, reshape the physical habitat and makes available previously unavailable resources. As such processes, and communities change along and patterns are created in time and space. Ecological models incorporating hydrodynamics and trophic structure are poised to serve as thinking pads allowing discovering and understanding patterns in different time and space scales of aquatic ecosystems. In lake ecosystem simulations, the horizontal spatial heterogeneity of the phytoplankton and the hydrodynamic processes are often neglected. Our simulations showed that it is important to consider this spatial heterogeneity in large lakes, as the water quality, community structures and hydrodynamics are expected to differ significantly between the littoral and the pelagic zones, and between differently shaped lakes. Especially for prediction of the water quality (including the variability due to wind) in the littoral zones of a large lake, the incorporation

stronger top-down control on the phytoplankton in this part of the lake.

communities (Cardoso & Motta Marques, 2004a, 2004b, 2004c). The rate of change in the phytoplankton was very high, indicating the occurrence of intense, rapid environmental changes, mainly in spring.

Marked changes in the spatial and temporal gradients of the plankton communities occurred during the seasons of the year, in response to resuspension events induced by the wind (Cardoso & Motta Marques, 2009). These responses were most intense precisely at the sites where the fetch was longest. The increase in changes occurred as the result of population replacements in the plankton communities. Resuspension renders diatoms and protists dominant in the system, and they are replaced by cyanobacteria and rotifers when the water becomes calm again (Cardoso & Motta Marques, 2003, 2004a, 2004b). Thus, diatoms and protists were the general indicator groups for lake hydrodynamic, with fast responses in their spatial distribution. Wind-induced water dynamics acted directly on the plankton community, resuspending species with a benthic habit.

In Itapeva Lake, water level and water velocity induced short-term spatial gradients, while wind action (affecting turbidity, suspended solids, and water level) was most strongly correlated with the seasonal spatial gradient (Cardoso & Motta Marques, 2009). In Lake Mangueira, water level was most strongly correlated with the seasonal spatial gradient, while wind action (affecting turbidity, suspended solids, and nutrients) induced spatial gradients.

The Canonical Correspondence Analysis (CCA) suggested that some aspects of plankton dynamics in Itapeva Lake are linked to suspended matter, which in turn is associated with the wind-driven hydrodynamics (Cardoso & Motta Marques, 2009). Short-term patterns could be statistically demonstrated using CCA to confirm the initial hypothesis. The link between hydrodynamics and the plankton community in Itapeva Lake was revealed using the appropriate spatial and temporal sampling scales. As suggested by our results, the central premise is that different hydrodynamic processes and biological responses may occur at different spatial and temporal scales. A rapid response of the plankton community to wind-driven hydrodynamics was recorded by means of the sampling scheme used here, which took into account combinations of spatial scales (horizontal) and time scale (hours).

In both lakes, the central zone of the lake takes on intermediate conditions, sometimes closer to the North part and sometimes closer to the South, depending on the duration, direction and velocity of the wind. This effect is very important for the horizontal gradients evaluated, in relation to the physical and chemical water conditions as well as to the plankton communities. In Lake Mangueira, the South zone is characterized by high water transparency whereas the North zone is more turbid, because the latter is adjacent to the wetland and is influenced by substances originated from the aquatic macrophyte decomposition. In Itapeva Lake it was not possible to distinguish such clear spatial differences. The spatial variation is directly related to wind action, because the lake is smaller and shallower than Lake Mangueira. In addition, the prevailing NE winds and the influence of the Três Forquilhas River on Itapeva Lake make the central zone often similar to the South part. The high turbidity in Itapeva Lake is an important factor affecting the composition and distribution of the plankton communities. However, in Lake Mangueira the marked spatial differences between the North and South zones were important for the composition and distribution of the plankton, and the influence of the wind was more evident in the Center zone than in the two ends of the lake.

In Lake Mangueira, wind-driven hydrodynamics creates zones with particular water dynamics (Fragoso Jr. et al., 2008). The velocity and direction of currents and water level changed quickly. Depending on factors such as fetch and wind, areas dominated by downwelling and upwelling could be identified in the deepest parts. We observed a significant horizontal spatial heterogeneity of phytoplankton associated with hydrodynamic patterns from the south to the north shore (littoral-pelagic-littoral zones) over the winter and summer periods. Our results suggest that there are significant horizontal gradients in many variables during the entire year. In general, the simulated depth-averaged chlorophyll-a concentration increased from the pelagic to the littoral zones. This indicated that a higher zooplankton biomass can exist in the littoral zones, leading, eventually, to stronger top-down control on the phytoplankton in this part of the lake.

Moreover, as expected for a wind-exposed shallow lake, Lake Mangueira did not show marked vertical gradients. The field campaigns showed that the lake is practically unstratified, emphasizing the shallowness and vertical mixing caused by the wind-driven hydrodynamics. This complete vertical mixing, as expected, was noted for both the pelagic and littoral zones. However, we are still of the opinion that incorporation of horizontal spatially explicit processes associated with the hydrodynamics is essential to understand the dynamics of a large shallow lake. The occurrence of hydrodynamic phenomena such as the seiches between the extreme ends, in a very long and narrow lake, is important, since seiches function as a conveyor belt, accounting for the vertical mixing and transportation of materials between the two ends of the lake and between the wetlands in the North and South areas in Lake Mangueira. Seiches was very important to explain much of the spatial changes in Itapeva Lake.
