Effects of <i>Carassius auratus</i> on water turbidity, nutrient levels and plankton biomass in shallow lakes: A mesocosm experiment

Abstract

鲫（Carassius auratus）是我国各类淡水水体的优势鱼类之一.作为底栖杂食性鱼类，一方面，鲫可以通过排泄和扰动沉积物影响湖泊营养和光照水平，通过上行效应促进浮游植物生长；另一方面，鲫也可以捕食浮游动物，通过下行控制影响藻类生长以及营养盐循环.对于浅水湖泊，两种途径对于生态系统影响的相对重要性仍有待研究.本研究设计了一个两因素户外中宇宙实验，通过在沉积物表面添加隔网的方式，比较两种情况下（能、否接触沉积物），鲫对水体浊度、营养盐和浮游生物生物量的影响.实验在16个大型钢化玻璃桶（400 L）中进行，持续36 d（2019年8月6日-9月11日）.研究结果表明：1）在能接触沉积物的条件下，鲫显著促进了沉积物再悬浮，表现为水体的总悬浮物（TSS）和无机悬浮物（ISS）浓度大幅升高；水体的光衰减系数（K_d）增加，总氮（TN）和总磷（TP）浓度明显升高；2）在不能接触沉积物的条件下，鲫对水体悬浮物（TSS和ISS）浓度和K_d的影响不明显，但是显著降低了水体TN和TP浓度；3）在两种情况下，鲫对浮游植物叶绿素a浓度以及浮游动物生物量的影响均不显著.本研究表明鲫只有在能够接触沉积物的条件下，才会显著提高水体浊度和营养水平.因此，在缺乏沉水植被的浅水湖泊中，鲫扰动沉积物产生的上行效应可能是其对生态系统产生负面影响的主要途径.;In China, crucian carp (Carassius auratus) are dominant fish species in various freshwater ecosystems. As an Omni-benthivorous fish, on one hand, crucian carp may affect water nutrient and turbidity levels via sediment disturbance and thereby enhance phytoplankton growth, a bottom-up route. On the other hand, crucian carp may also affect algal growth and nutrient cycling via predation on zooplankton and thereby reducing zooplankton grazing on phytoplankton, a top-down process. For shallow lakes, the relative importance of both mechanisms remains to need to be elucidated. Here, we designed an outdoor mesocosm experiment to compare the effects of crucian carp on water turbidity, nutrient and phytoplankton growth between mesocosms with (via adding a sieve above the sediments) and without sediment disturbance. The experiment was conducted using 16 armoured glass tanks (400 L) and lasted for 36 days (from August 6 to September 11 in 2019). The results showed that:1) in mesocosms with sediment disturbance (without sieve), crucian carp caused severe sediment resuspension, reflected by the large increases of total suspended solids (TSS) and inorganic suspended solids (ISS). Correspondingly, light attenuation efficient (K_d), total nitrogen (TN) and total phosphorus (TP) concentrations were significantly higher in mesocosms with crucian carp than ones without fish. 2) In mesocosms without sediment disturbance (sieve present), the effects of crucian carp on suspended solids (TSS, ISS) and K_d were both insignificant. However, TN and TP concentrations were significantly reduced by crucian carp. 3) no significant effects of crucian carp on chlorophyll-a concentration and zooplankton biomass were observed. Our study suggested that crucian carp promote water turbidity and nutrient levels only when they could access the sediments. Thus, in shallow lakes without submerged macrophytes, bottom-up control may be the main mechanism of crucian carp affecting water quality adversely.