Spatial and annual variability of environmental and phytoplankton indicators in Lake Võrtsjärv: implications for water quality monitoring

• Published in: Estonian journal of ecology Vol. 61; no. 4; pp. 227 - 246
• Main Authors: Nõges, P, Tuvikene, L
• Format: Journal Article
• Language: English
• Published: Tallinn Teaduste Akadeemia Kirjastus (Estonian Academy Publishers) 01.01.2012
• Subjects: Annual variations; Freshwater; Phytoplankton; Water monitoring; Water quality management; Estonia, Vortsjarv L
• ISSN: 1736-602X; 1736-7549
• DOI: 10.3176/eco.2012.4.01

Monitoring purposes determine the selection of variables, location of sampling sites, and sampling frequency. The selection should provide the best signal to noise ratio for the parameters of interest. For trend and surveillance monitoring, the deepest point of a lake, where different inputs to the lake are integrated, is frequently selected. However, the representativeness of a single site is often questioned, especially for large lakes. Based on data collected from 10 sampling points during 11 survey expeditions in August 2001-2011 to the large shallow Lake Vortsjarv, Estonia, we studied the spatial and annual variability of environmental and phytoplankton variables and analysed the representativeness of a permanent sampling station for the whole lake conditions. The two southernmost stations under the influence of the main tributary deviated clearly from the homogeneous group of the other eight stations, which we termed 'Vortsjarv Proper'. Among the stations of Vortsjarv Proper, the year-to-year variability dominated strongly over the spatial variability, the latter being almost negligible for most of the variables. Surface water temperature and water level explained approximately half of the total variability in parameters commonly used in ecological status assessment of lakes. This has serious implications for using these variables to detect human impacts in Vortsjarv. Our study showed that the deep sampling site, which was characterized by the lowest average variability of the parameters measured and was representative of more than 90% of the lake aquatory, possesses all necessary qualities required of a permanent surveillance monitoring station.Original Abstract: Seire eesmargid maaravad, millised naitajad, proovikohad ja milline proovivotu sagedus on meid huvitava info saamiseks koige okonoomsemad. Ulevaate- ja uuriva seire kohaks valitakse tavaliselt jarve sugavaim koht, eeldades, et seal avalduvad jarve kui tervikut iseloomustavad omadused koige paremini. Suurte jarvede puhul tekitab aga ainult uhe proovipunkti kasutamine sageli kusimuse, kas see on kogu jarve iseloomustamiseks piisavalt esinduslik. Uurisime suure, madala Vortsjarve kumne proovikoha augustikuisel seirel aastatel 2001-2011 saadud keskkonnanaitajate ja futoplanktoni andmete ruumilist ning aastatevahelist varieeruvust, et selgitada, kas uks pusivaatluskoht on kogu jarve iseloomustamiseks kullalt esinduslik. Kaks koige lounapoolsemat proovikohta, mis on Vaikese Emajoe mojualas, eristusid selgelt ulejaanud kaheksast proovikohast. Viimaste hulgas oli aastatevaheline varieeruvus markimisvaarselt suurem kui ruumiline. Veetaseme ja pinnakihi temperatuuri muutustega oli Vortsjarves seletatav ligikaudu pool jarvede okoloogilise seisundi iseloomustamiseks tavaliselt kasutatavate parameetrite kogumuutlikkusest. See tekitab viimaste kasutamisel inimmoju hindamiseks Vortsjarvele tosiseid probleeme. Vorreldes teiste proovipunktidega on jarve sugavas kohas asuva proovipunkti naitajate keskmine varieeruvus madalaim. See proovipunkt sobib iseloomustama 90% jarve akvatooriumist ja selle kasutamine Vortsjarve ulevaateseire statsionaarse seirekohana on oigustatud.