User:Lmnocarp/Lago Grande de Curuaí

Background
Lago Grande de Curuaí is the major lake in the "Várzea do Lago Grande de Curuaí" floodplain. It is a Amazon floodplain lake near Óbidos in the state of Pará, Brazil, about 900 km from the Amazon river mouth.

Floodplain Lake
The floodplain is composed of over 30 interconnected lakes formed by quaternary sandy sediments and are connected to the Amazon mainstream by several temporary and permanent channels. The várzea is composed of white-water lakes (lakes with high suspended sediment loads) and black-water lakes (lakes with high concentrations of dissolved humic acids and low concentrations of suspended sediment). Lago Grande de Grande Curuaí itself is designated as a white-water lake, the main channels that connect it to the Amazon permanently are Foz Sul (FS) and Foz Norte (FN).

Seasonal changes and Limnology
Because it is a floodplain lake, area of the lake changes dramatically based on the seasons. The lake covers an area of about 359 km2 and is located between 56.10°W and 55.00°W from upstream to downstream, and 2.3°S and 1.9°S. The flooded areas of the lake ranges between 575 km2 and 2090 km2 depending on the water levels. The water shed of the várzea is 3660 km2 The maximum and minimum water levels recorded at the Curuaí gauging station from 1999 to 2006 are 3.03 m and 9.61m respectively with a maximum water level variation of 6.7m. The storage volume of the floodplain ranges from 11.9 km2 during the low water season and 116.2 km2 during high water season. The average depth of the lake varies based on the season as well. During the high water season the average depth was 10.9 m and in low water the average depth was 4.9 m.

The seasons of high and low water are well defined and correspond approximately with the annual water-level fluctuations of the Amazon River. The lake follows cycles of high water and low water with the high water peak occurring during the months of May to July and the low water peak occurring between October to December. Water flows from the Amazon into the floodplain in January, both in and out from February to April and flows out from late April to October. The flow between the Amazon and the floodplain at the FS and FN channels are directed by differences in rates of water-level rise. The mean annual hydrologic residence time of the lake is about 3 months.

Nutrients and sediments from the Amazon River as well as the connectivity of lakes change ecological conditions. As a result the timing and seasons of algal blooms, tree seed and fruit dispersal are synchronized with the cycles of flooding. This in turn has an effect on the feeding habits of fish as well as the growth of aquatic macrophytes.

Lake Ecology
The Lago Grande de Curuaí floodplain lake, like most Amazonian várzeas is characterized by a complex distribution of 4 major plant communities: algae (periphyton and phytoplankton), aquatic herbaceous plants, terrestrial herbaceous plants and várzea forest. In the lake rotifers were found to be dominant in terms of species richness and copepods were found to be dominant in individual abundance according to a study done in 2015.

Lake Water Quality
During the hydrological year of 2003 to 2004 the Lago Grande de Curuaí floodplain was classified as a supereutrophic system. The levels of dissolved oxygen (DO) and chlorophyll-α were deemed as unsuitable for human and animal supply during the low water season

Organic Carbon Studies
Amazon floodplains are very important in sequestering sedimentary organic carbon and as such have been the topic of many studies. Lago Grande de Curuaí was the site of a study focusing on the seasonal pattern of the origin, composition and concentration of organic matter of Amazonian floodplains. Moreira-Turcq et al. 2013 looked at carbon in the system during rising, high, falling, and low water periods over 7 years (1999–2006). During the rising period the main source of organic matter is from the Amazon River, but during the falling period the main source is from in situ production dominated by macrophytes and phytoplankton, including cyanobacteria. The study found that organic matter in the floodplain was characterized with low carbon to nitrogen ratios (C:N) and high chlorophyll a concentrations. In addition, the carbon δ13C signature had seasonal variations with more negative values corresponding to the high water period. Based on their elemental analysis of particulate organic matter they determined that Lago Grande de Curuaí, and by extent floodplain lakes in the Amazon Basin, have large amounts of phytoplankton and macrophyte primary production which is then exported to the main Amazon River. Thus, Lago Grande de Curuaí was found to be an important source of labile organic matter to the Amazon during falling water season, a contrast to previous studies that have said that floodplains are a negligible source of carbon.

Water and sediments are constantly being transported between the main stem of the Amazon and its floodplain. However, the rates at which sediments are transferred to and from floodplains are not well known. In addition, tropical floodplains are the largest source of atmospheric methane, with a contribution of 60%. According to some studies the mass balance of sediments and carbon show a loss indicating a sink in váreza systems. Moreira-Turcq et al. 2005 examined the dynamics of biogeochemical cycling of organic carbon in Lago Grande de Curuaí. They found the same findings as the Moreira-Turcq et al. 2013, but also that 70-90% of total organic carbon (TOC) in the váreza was dissolved organic carbon (DOC), but that particulate organic carbon was mostly exported to the river.

Zooplankton
Another study wanted to look at how zooplankton environmental characteristics and community structure were affected by the seasonal variations of flooding. They did this by sampling at 20 sites throughout the Lago Grande de Curuaí floodplain. They found that instead of the extreme heterogeneity that they expected between sites and flooding periods due to reducing and increasing connectivity of water, the sites were homogeneous. They attributed this to the fact that the Amazon River has a strong flow that likely quickly homogenizes any differing environmental characteristic. In the study they found that testates amoeba showed high richness, but low abundance and exhibited heterogeneity in dispersion among sites. Copepods however, did not express heterogeneity which could be due to their longer lifespans. Cladocera and rotifers both exhibited heterogeneity in between sites and hydrological periods, with cladocerans exhibiting more heterogeneity in dispersion during the falling water period and rotifers during the rising water period. However, the abundance of rotifers was 6 times higher during the falling water period, implicating that changes in primary productivity could be the determining factors.

Bacterioplankton Community Composition
In a spatial and temporal assessment of bacterioplankton community composition (BCC) in Lago Grande de Curuaí during different hydrological periods, there was found to be a high degree of novelty with more than half of the bacterioplankton (operational taxonomic units (OTUs)) could not be classified on a genus level. Spatial habitat variability and flood pulse were the factors responsible for the free floating BCC. The main finding of the study however was that variation in space played more of a role in determining BCC than temporal variation, though both were important drivers in the váreza system.