Shallow Lakes

Complementary analysis of satellite mission data (altimetry, Moderate Resolution Imaging Spectroradiometers (MODIS)) and climate fields over 2000–2009 was conducted to investigate the variability of the water cover surfaces and the geographical source of water inflowing into the Lake Poopó system. The results suggest that over the time span of 2000–2009 a great part of the variability of the Poopó system originates from geographic sources other than Lake Titicaca. Possible alternative causes include climate change inducing increased temperatures and greater evaporation rates along the Altiplano; increased glacier and snow melting over the Andes Cordillera; and a potential increased anthropogenic water use (such as irrigation, minery, etc.) throughout the path of the Desaguadero River (which connects lakes Titicaca and Poopó). This change in the hydrology of this region could lead to a collapse of water supplies and endanger the already fragile Poopó system as well as the regional socioeconomic system, which closely depends upon it.

Zeekoevlei is the largest freshwater lake in
South Africa and has a century-long history of
anthropogenic impact that caused hyper-eutrophic
conditions. We used biomarkers (alkanes and pigments),
stable isotopes (d13C and d15N), rates of
primary palaeoproduction and total inorganic carbon
(TIC) accumulation rates in the lake sediments to
investigate changes in plankton and macrophyte
communities in response to anthropogenic activities
in this shallow lake. Specific alkanes and pigment
(chlorophyll a, b,b-carotene, echinenone, fucoxanthin
and zeaxanthin) concentrations in lake waters indicated
the present-day hyper-eutrophic condition and
seasonal fluctuations of cyanobacteria, zooplankton
and diatom populations. Eutrophic conditions were
initiated in the lake with the start of recreational
activities and construction of a sewage treatment plant
in the early 1920s. The lake transformed from a
eutrophic to a hyper-eutrophic waterbody following
damming, pondweed eradication and accelerated
catchment-derived nutrient input. The change in lake
trophic state was recorded by a sharp decline in the
terrestrial to aquatic ratio (TAR) of specific n-alkanes,
low carbon preference index (CPI) and increased d13C
values in the sediment core. In addition, the aquatic
macrophyte n-alkane proxy (Paq) values (1) indicated
a slow takeover by floating macrophytes after
the eradication of submerged pondweeds in 1951.
Elevated n-alkane, total alkane and pigment
(chlorophyll a, b,b-carotene, zeaxanthin and
zeaxanthin to b,b-carotene ratio) concentrations, low
d15N values and low TIC accumulation rates in the
upper middle section of the core indicated the
beginning of intense cyanobacterial blooms after the
dredging in 1983. Although the cyanobacterial population
has decreased in recent years, hyper-eutrophic
conditions are reflected by low CPI<0.04 and TAR<1
values at the top of the sediment core.

Lake Vela is a polymictic shallow lake exhibiting some characteristics typical of an advanced trophic state, namely, the permanently turbid water, the reduction in biodiversity, and the recurrent occurrence of Cyanobacteria blooms, which occasionally lead to large fish kills. This study was carried out in order to understand the seasonal variation of Lake Vela’s phytoplankton and cladoceran communities and their interactions under advanced eutrophic conditions. When comparing our results with general models of plankton succession observed in other temperate and tropical lakes we found some coherence between several seasonal events. Our results suggest that phytoplankton is mainly regulated by nutrients (“bottom-up” effect). However, the warm mediterranean temperatures had an important role in the phytoplankton succession, being responsible for the rapid and intense Cyanobacteria development in spring and summer. Our work also demonstrated that phytoplankton is one of the main factors responsible for the seasonal structure of the community of cladocerans, which are well related to changes in algae diversity and abundance, being the Cyanobacteria having major impact. Moreover, the occurrence of a massive Cyanobacteria bloom during the study, which induced anoxia and consequent fish kill, enhanced the structuring role that fish have on the cladoceran seasonal succession and the effects of this episode in the normal seasonal succession of plankton.

1. We studied the effects of increased water temperatures (0–4.5 °C) and nutrient enrichment on the stoichiometric composition of different primary producers (macrophytes, epiphytes, seston and sediment biofilm) and invertebrate consumers in 24 mesocosm ecosystems created to mimic shallow pond environments. The nutrient ratios of primary producers were used as indicative of relative nitrogen (N) or phosphorus (P) limitation. We further used carbon stable isotopic composition (δ13C) of the different primary producers to elucidate differences in the degree of CO2 limitation.2. Epiphytes were the only primary producer with significantly higher δ13C in the enriched mesocosms. No temperature effects were observed in δ13C composition of any primary producer. Independently of the treatment effects, the four primary producers had different δ13C signatures indicative of differences in CO2 limitation. Seston had signatures indicating negligible or low CO2 limitation, followed by epiphytes and sediment biofilm, with moderate CO2 limitation, while macrophytes showed the strongest CO2 limitation. CO2 together with biomass of epiphytes were the key variables explaining between 50 and 70% of the variability in δ13C of the different primary producers, suggesting that epiphytes play an important role in carbon flow of temperate shallow lakes.3. The ratio of carbon to chlorophyll a decreased with increasing temperature and enrichment in both epiphytes and seston. The effects of temperature were mainly attributed to changes in algal Chl a content, while the decrease with enrichment was probably a result of a higher proportion of algae in the seston and epiphytes.4. Macrophytes, epiphytes and seston decreased their C : N with enrichment, probably as an adaptation to the different N availability levels. The C : N of epiphytes and Elodea canadensis decreased with increasing temperature in the control mesocosms. Sediment biofilm was the only primary producer with lower C : P and N : P with enrichment, probably as a result of higher P accumulation in the sediment.5. Independently of nutrient level and increased temperature effects the four primary producers had significantly different stoichiometric compositions. Macrophytes had higher C : N and C : P and, together with epiphytes, also the highest N : P. Seston had no N or P limitation, while macrophytes and epiphytes may have been P limited in a few mesocosms. Sediment biofilm indicated strong N deficiency.6. Consumers had strongly homeostatic stoichiometric compositions in comparison to primary producers, with weak or no significant treatment effects in any of the groups (insects, leeches, molluscs and crustaceans). Among consumers, predators had significantly higher N content and lower C : N than grazers.

A multi-proxy approach study (cladocerans, diatoms, geochemistry, plant macrofossils, pollen), was performed on a sediment core from Lake Vrana (Vransko Jezero), a large and deep karstic lake on the northern Adriatic island of Cres, Croatia. Considerable lake-level changes occurred during the last approx. 16,000 years. The stratigraphic evidence suggests that periods of enhanced pre- cipitation and the post-LGM rise in

A long-term food web manipulation experiment was started in 1999 with monitoring in the eutrophic shallow Major Lake (area 11 ha, mean depth 1.1 m). In 2000, studies were continued with removal of 204 kg ha−1 of fish and restocking with 134 kg ha−1 of cyprinids. The removal of about 50% of cyprinid had a quite considerable impact on almost all of the observed parameters. Summer mean chlorophyll a values in 2000 decreased by 43%, phytoplankton community changed from being dominated by filamentous blue-greens causing blooms in 1999 to a much more diverse community with, e.g., green algae, dinoflagellates and cryptomonads in the year following fish removal. The zooplankton community showed the typical composition of lakes with very high predation pressure of omnivorous cyprinid fish leading to a decrease in both mean body length and fecundity of daphnids.

1. Seasonal relationships between macrophyte and phytoplankton populations may alter considerably as lakes undergo eutrophication. Understanding of these changes may be key to the interpretation of ecological processes operating over longer (decadal-centennial) timescales.2. We explore the seasonal dynamics of macrophytes (measured twice in June and August) and phytoplankton (measured monthly May–September) populations in 39 shallow lakes (29 in the U.K. and 10 in Denmark) covering broad gradients for nutrients and plant abundance.3. Three site groups were identified based on macrophyte seasonality; 16 lakes where macrophyte abundance was perennially low and the water generally turbid (‘turbid lakes’); 7 where macrophyte abundance was high in June but low in August (‘crashing’ lakes); and 12 where macrophyte abundance was high in both June and August (‘stable’ lakes). The seasonal behaviour of the crashing and turbid lakes was extremely similar with a consistent increase in nutrient concentrations and chlorophyll-a over May–September. By contrast in the stable lakes, seasonal changes were dampened with chlorophyll-a consistently low (<10–15 μg L−1) over the entire summer. The crashing lakes were dominated by one or a combination of Potamogeton pusillus, Potamogeton pectinatus and Zannichellia palustris, whereas Ceratophyllum demersum and Chara spp. were more abundant in the stable lakes.4. A long-term loss of macrophyte species diversity has occurred in many shallow lakes affected by eutrophication. One common pathway is from a species-rich plant community with charophytes to a species-poor community dominated by P. pusillus, P. pectinatus and Z. palustris. Such compositional changes may often be accompanied by a substantial reduction in the seasonal duration of plant dominance and a greater tendency for incursions by phytoplankton. We hypothesise a slow-enacting (10–100 s years) feedback loop in nutrient-enriched shallow lakes whereby increases in algal abundance are associated with losses of macrophyte species and hence different plant seasonal strategies. In turn such changes may favour increased phytoplankton production thus placing further pressure on remaining macrophytes. This study blurs the distinction between so-called turbid phytoplankton-dominated and clear plant-dominated shallow lakes and suggests that plant loss from them may be a gradual process.

1. Palaeolimnology and contemporary ecology are complementary disciplines but are rarely combined. By reviewing the literature and using a case study, we show how linking the timescales of these approaches affords a powerful means of understanding ecological change in shallow lakes.2. Recently, palaeolimnology has largely been pre-occupied with developing transfer functions which use surface sediment-lake environment datasets to reconstruct a single environmental variable. Such models ignore complex controls over biological structure and can be prone to considerable error in prediction. Furthermore, by reducing species assemblage data to a series of numbers, transfer functions neglect valuable ecological information on species’ seasonality, habitat structure and food web interactions. These elements can be readily extracted from palaeolimnological data with the interpretive assistance of contemporary experiments and surveys. For example, for one shallow lake, we show how it is possible to infer long-term seasonality change from plant macrofossil and fossil diatom data with the assistance of seasonal datasets on macrophyte and algal dynamics.3. On the other hand, theories on shallow lake functioning have generally been developed from short-term (<1–15 years) studies as opposed to palaeo-data that cover the actual timescales (decades–centuries) of shallow lake response to stressors such as eutrophication and climate change. Palaeolimnological techniques can track long-term dynamics in lakes whilst smoothing out short-term variability and thus provide a unique and important means of not only developing ecological theories, but of testing them.4. By combining contemporary ecology and palaeolimnology, it should be possible to gain a fuller understanding of changing ecological patterns and processes in shallow lakes on multiple timescales.