The phytoplankton associations in HRB and Dishui Lake
Twelve phytoplankton associations in HRB were identified by combinations
of two to three taxa (Table 5) based on support>=50%, and
confidence values of >=0.8. All associations can be divided
into three types: (1) The flagellate algae association (R01-1, R01-2,
R01-9): pollution-tolerant species (e.g., Cryptomonas ovata andCryptomonas erosa) co-existed; mixotrophic chrysophytes (e.g.,Chrumulina sp.) was observed to occur with Cryptomonas
erosa , Chroomonas acuta , Chlamydomonas globosa from
different taxon groups. (2) The diatoms with the flagellate algae
association (R03-2, R05-2, R06-2, R07-2, R08-2, R10-2, R11-2 and R12-2):
benthic diatoms with motility (e.g., Nitzschia palea ,Navicula cryptocephala) frequently co-occurred with the
cryptophytes. It should be noted that a small centric planktonic diatom
(Cyclotella meneghiniana ) also occurred with the flagellate
algae. (3) The diatoms association (R04-3: Nitzschia palea andCyclotella meneghiniana ).
Fifteen phytoplankton associations in Dishui Lake were identified by
combinations of two to three taxa (Table 5). These associations can be
divided into three types: (1) The flagellate algae association (L11-1):
mixotrophic chrysophyte Chrumulina pygmaea was observed to occur
with Chlamydomonas globosa . (2) The
mixed
association-diatoms or colonial green algae with the flagellate algae
association: this association can be further divided into three smaller
associations including mixotrophic chrysophyte Chrumulina pygmaeawith diatoms or green algae (L03-2, L07-2, L09-2, and L12-2),Chlamydomonas globosa with diatoms or green algae (L01-2, L02-2,
L04-2, L05-2, L08-2, L10-2 and L13-2), and Chromulina pygmaea andChlamydomonas globosa with diatoms or green algae (L14-2 and
L15-2). (3) The diatoms with colonial green algae association (L06-3).
Relationships
between phytoplankton associations and environmental variables
We analyzed the phytoplankton assemblages characterized with 12
phytoplankton associations in HRB using detrended correspondence
analysis (DCA). DCA results showed that the maximum gradient length of
the four axes was 2.63. Subsequently, we selected a redundancy analysis
(RDA) to detect the relationship between phytoplankton associations and
environmental factors (Fig. 2). Approximately 12% of the variance in
phytoplankton associations can be explained by environmental factors
(axis 1: 8%, axis 2: 3%). Forward selection in RDA identified six
significant environmental factors (Fig. 2).
Turbidity
was positively correlated with axis 1, TN/TP ratio was negatively
correlated with axis 1; Conductivity and TN positively correlated with
axis 2, stream order negatively correlated with axis 2. Most of the
mixed associations had a positive relationship with TN and turbidity
except that association 3 displayed a positive relationship with
conductivity and negative correlation with turbidity (Fig. 2).The
flagellate algae association had a positive relationship with turbidity,
DO and stream size while the diatom association was positively
associated with TN(Fig. 2).
We used the 15 phytoplankton associations in Dishui Lake for detrended
correspondence analysis (DCA) with the maximum gradient length of the
four axes as 1.55. RDA showed that 32%
variance in phytoplankton
associations can be explained by environmental factors: axis 1: 27%,
axis 2: 4.8%. Forward selection in RDA identified three significant
environmental factors (Fig. 3). Salinity and transparency were
negatively correlated with axis 1; pH was negatively correlated with
axis 2. The mixed associations including Chromulina had a
positive relationship with pH and transparency while the mixed
associations including Chlamydomonas had a negative relationship
with pH (Fig. 3). The flagellate associations positively correlated with
salinity (Fig. 3).