Study On Nutrient Limitation Of Phytoplankton In Dianshan Lake

Lake eutrophication and the outbreak of algal bloom subsequently become major environmentalissues facing in China and over the world. In particular, as the urban drinking water resource, thelake eutrophication exerts a serious threat on the security of the water supply. This paper, in order toprotect the safety of water supply in Shanghai, taking the Dianshan Lake as the research object, carryout the study about the nutrient limiting factors of the phytoplankton growth and its relationship withthe nutrients, based on the nutrient levels over the history (year 1986-2007) and the current status ofwater quality (2008-2009).(1) By analyzing the water quality data during the past 22 years (1986-2002), the ecosystem ofLianshan Lake have undertaken great changes, with the water eutrophication level from poor tosevere. The water parameters such as TN, TP and COD experience continuous increase, with DOand transparency decrease as a result. The dominant species of phytoplankton transform gradually tonutrient-rich, the proportion of diatom gradually decrease while the green and blue algal increasewhich leads the species of phytoplankton and the diversity decrease sharply.(2) The Nutrient Enrichment Bioassay in Laboratory is conducted to study the nutrientsrestriction of phytoplankton growth. The result shows that the enrichment of PO₄^3--P during Oct-Deccan increase the relatively coefficient, phytoplankton biomass and growth rates, while theenrichment of NO₃ - N and NH₃-N have little impact. But the addition of P and NO₃-N can increasethe biomass significantly. However, the main effect analysis shows that there is no interactionbetween P and NO₃-N, with the P promote the growth of phytoplankton only, P is the limiting factorduring Oct-Dec. In Feb-Mar, for the sufficient N and P content in the lake which has no significanteffect on the growth of phytoplankton, nutrient is not a major factor during this period.(3) Nutrient Enrichment Bioassay on site is conducted to show that the enrichment of PO₄^3--Pduring April-June can increase the relatively coefficient, phytoplankton biomass and growth rates,while the enrichment of NO₃ - N and NH₃-N have little impact. It indicates that P promote the growth of phytoplankton during April-June which is the limited factor for the growth ofphytoplankton; However, during July-Sep, the addition of P has little impact on the growth ofphytoplankton, while the addition of NO₃ - N and NH3-N can increase the relatively coefficient andphytoplankton biomass that is the limited factor for the growth of phytoplankton this period. Thelimiting factor of phytoplankton growth experiences a changing all over the year, from P-limitation(Oct-Dec) to no limitation (Feb-March) then N-limitation (July-Sep).(4) The analysis of the water quality data and ecological data during 2008-2009 is conducted toshow that: TP has two peaks in Oct-March and May-July, respectively; while the peaks of TN inJan-March and June-July, respectively and the Chlorophyll-a have two peaks in March-May andJune-July, respectively. The distribution of nutrients and chlorophyll-a is staggered, and theproportion of dissolved nutrients in the total nutrients change greatly, so the (NH₃-N+NO₃^--N)/PO₄^3--P can be a better parameter used to judge the nutrients limited status compared to TN/TP.When (NH₃-N+NO₃^--N) /PO₄^3--P≤57, the growth of phytoplankton can be limited by N; while the57≥(NH₃-N+NO₃^--N) /PO₄^3--P≤71, the growth of phytoplankton is not limited by either N or P;while (NH₃-N+NO₃^--N) /PO_<sup>3--P≤71, the growth of phytoplankton can be limited by P.